Which Materials in a Turbomachinery Lubrication System Attract More Varnish — Chemistry, Metallurgy, and Surface Polarity

In turbomachinery lubrication systems, varnish does not deposit randomly. It preferentially deposits on surfaces where the surface energy, polarity, temperature profile, flow regime, electrostatic condition, and metallurgy/oxide chemistry make the surface attractive to polar degradation products in the oil.

This is why in real machines we often see varnish heavily on:

journal bearing shells, thrust bearing pads, Babbitt surfaces, servo valves, control valves, reservoir internals, cooler tubes, small clearances, strainers, filter housings, and dead zones — while other large surfaces may look almost clean.

The important point is this:

Turbine oil varnish is mainly made from polar oxidation and degradation products.
These products are chemically attracted to polar, high-energy, oxidized, reactive, or electrically charged surfaces.

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1. What Is Varnish Chemically?

Turbine oil is mostly non-polar hydrocarbon base oil. During service, oxidation, thermal stress, microdieseling, electrostatic discharge, additive depletion, and contamination generate more polar molecules.

These include:

Organic acids
Carboxylic acids, low-molecular-weight oxidation products, acidic degradation species.

Aldehydes and ketones
Intermediate oxidation products from hydrocarbon chain breakdown.

Esters, lactones, peroxides, hydroperoxides
Oxidation intermediates that can further polymerize.

Resinous oligomers and polymers
Higher-molecular-weight oxidation products formed by condensation and polymerization reactions.

Additive degradation products
Especially from phenolic and aminic antioxidants, rust inhibitors, antiwear additives, and metal passivators.

Inorganic and organometallic contaminants
Iron oxides, copper compounds, tin/lead species from bearing metallurgy, silica, dust, rust, and process contamination.

These materials are far more polar than the original base oil. So they behave differently from the oil. They can stay dissolved at high temperature, but when solubility decreases or surface attraction is strong, they leave the oil phase and deposit.

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2. Why Polar Varnish Precursors Are Attracted to Surfaces

Fresh turbine oil is mostly non-polar. Varnish precursors are polar. Surfaces inside the lubrication system are often polar too, especially when oxidized.

The attraction mechanisms include:

2.1 Acid–base interaction

Oxidized metal surfaces contain hydroxyl groups and metal oxides. Acidic degradation products in the oil can chemically interact with these surface sites.

Example:

Iron surface exposed to oil and oxygen:

Fe → FeO / Fe₂O₃ / Fe₃O₄ surface layer

These oxide layers are polar and can interact with carboxylic acids and other polar oxidation products.

So instead of remaining suspended in oil, the polar molecule prefers the metal oxide surface.

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2.2 Hydrogen bonding

Many varnish precursors contain oxygen-containing functional groups:

–COOH
–OH
C=O
–OOH
–COOR

These can hydrogen-bond with hydroxylated metal oxide surfaces, water films, rust layers, or polar polymeric surfaces.

This is one reason a surface with slight oxidation, moisture history, or aged coating can become a strong varnish collector.

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2.3 Dipole–dipole attraction

Polar varnish molecules have molecular dipoles. Oxidized metal surfaces also have polar sites.

The result is dipole-driven adsorption.

This is especially important in very small clearances such as:

servo valves,
pilot valves,
control oil orifices,
bearing feed grooves,
thrust pad edges,
hydraulic actuators.

Even a thin molecular layer can reduce clearance or increase friction.

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2.4 Electrostatic attraction

Turbine oil systems can generate electrostatic charge, especially across fine filters, high flow velocities, low-conductivity Group II/III oils, and certain filter media.

Charged varnish particles or polar soft contaminants can be attracted to oppositely charged surfaces.

This is one reason varnish can appear heavily in:

filter housings,
downstream of filters,
control valves,
reservoir internals,
small-bore piping.

Electrostatic discharge can also create localized thermal cracking, forming darker carbonaceous material that may appear on MPC patches as black or dark grey material.

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2.5 Surface energy and wetting

High-energy surfaces attract polar contaminants more strongly than low-energy surfaces.

In simple language:

Clean, oxidized, rough, metallic, reactive surfaces = high surface energy = more varnish attraction.

Low-energy, smooth, chemically inert surfaces are less attractive.

This is why surface finish, metallurgy, oxide layer, cleanliness, and temperature history matter.

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3. Ranking of Materials That Usually Attract More Varnish

Highest varnish attraction

1. Oxidized carbon steel and cast iron

Carbon steel and cast iron are very common in reservoirs, piping, bearing housings, cooler shells, valve blocks, and structural internals.

They are highly prone to forming iron oxide layers. These oxides are polar and reactive.

Common surface chemistry:

FeO
Fe₂O₃
Fe₃O₄
FeOOH

These oxide layers can adsorb acidic and polar oxidation products very strongly.

Why they attract varnish:

high surface polarity,
oxide layer formation,
rust and hydroxide sites,
micro-roughness,
acid-base interaction with organic acids,
ability to catalyze further oxidation.

In real turbomachinery systems, old carbon steel reservoirs and piping can become “varnish memory surfaces.” Even after oil replacement, these surfaces may release or re-adsorb degradation products depending on temperature and oil chemistry.

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2. Copper and copper alloys

Copper, brass, bronze, and copper-containing alloys can be very active in turbine oil degradation.

Common copper alloys in systems:

bronze bushings,
brass components,
copper cooler tubes,
copper-bearing alloys,
valve components,
instrument fittings.

Copper forms surface oxides:

Cu₂O
CuO

These oxides are polar and can adsorb polar oxidation products. More importantly, copper can catalyze oxidation reactions in oil.

Why copper alloys are critical:

copper catalyzes oil oxidation,
copper oxides are polar,
copper compounds can form organometallic deposits,
organic acids can react with copper surfaces,
copper corrosion products may become part of varnish.

Copper does not only “attract” varnish. It can also help create more varnish precursors.

That is why copper corrosion control and antioxidant health are very important in turbine oils.

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3. Babbitt bearing metals

Babbitt is widely used in journal and thrust bearings. Typical Babbitt materials are tin-based or lead-based alloys.

Common tin-based Babbitt may contain:

Sn
Sb
Cu

Common lead-based Babbitt may contain:

Pb
Sb
Sn

The Babbitt surface is soft, reactive, and often operates under high temperature, boundary/mixed lubrication zones, and high load.

Surface chemistry may involve oxides such as:

SnO
SnO₂
PbO
Sb₂O₃
CuO / Cu₂O

These oxides are polar. The surface may also have micro-smearing, fatigue marks, thermal distress, wiping, or polishing, all of which increase the ability of degradation products to attach.

Why Babbitt attracts varnish strongly:

polar oxide surface,
elevated operating temperature,
very thin oil film,
boundary lubrication zones during start/stop,
soft metal surface with micro-roughness,
high residence time of hot oil near the bearing,
possibility of local catalytic effect from copper/lead/tin compounds.

In the bearing photo you shared, the amber/brown staining pattern is exactly the kind of area where I would suspect polar oxidation products, thermal staining, and varnish-like deposits to interact with the bearing metal and oil film environment.

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4. Servo valve and control valve metallic surfaces

Servo valves are among the most sensitive components to varnish because they have:

very small clearances,
sharp edges,
spool/land geometry,
low leakage tolerance,
high surface finish,
critical movement,
often lower flow areas where deposits can stick.

Common materials include:

hardened stainless steel,
tool steel,
nitrided steel,
chrome-plated surfaces,
aluminum bronze,
special alloys.

Even stainless steel can attract varnish when the passive oxide layer is present.

Stainless steel surface oxide is mainly:

Cr₂O₃
Fe₂O₃
NiO traces

The chromium oxide passive layer is chemically stable but still polar. In tight-clearance components, even weak adsorption becomes a serious reliability issue.

Why servo valves suffer:

small deposit thickness causes big functional effect,
polar oxide surface,
low movement tolerance,
electrostatic attraction,
cooler local zones can reduce oil solubility,
deposit shear and re-deposition can cause stiction.

This is why varnish does not need to be thick in a servo valve. A very thin sticky film can already create sluggish response, hysteresis, or sticking.

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5. Cooler tube surfaces

Coolers are very important varnish locations because they introduce a temperature gradient.

Hot oil carries soluble varnish precursors. As oil is cooled, solubility decreases. Polar degradation products can come out of solution.

Common cooler materials:

copper alloy tubes,
brass tubes,
stainless steel tubes,
carbon steel shells,
titanium tubes in some applications.

Copper alloy coolers can be especially problematic because they combine:

metal catalytic activity,
polar oxide surface,
cooler temperature zone,
possible low-flow boundary layer,
water contamination risk.

This is a perfect environment for adsorption and precipitation.

A cooler may not be the root cause of varnish, but it can become a strong deposition point.

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6. Reservoir internals and painted/coated surfaces

Reservoirs are often underestimated. Your first photo shows exactly the type of internal zone where oil residence time, stagnant zones, dirty surfaces, rags, coating condition, air, water, and sludge can all influence varnish behavior.

Reservoir materials may include:

carbon steel,
painted steel,
epoxy-coated steel,
stainless steel internals,
mesh screens,
baffles,
suction strainers.

Painted or coated surfaces can become varnish collectors when the coating ages, absorbs polar molecules, or becomes micro-rough.

Epoxy coatings contain polar groups such as:

hydroxyl groups,
ether groups,
epoxide-related structures,
amine-cured networks.

These can interact with polar degradation products.

Reservoir varnish attraction is increased by:

dead zones,
low flow,
water settling,
rust,
aged coatings,
high air contact,
foam and entrained air,
dirty internal surfaces,
temperature cycling,
contaminated rags or maintenance debris.

The rags shown in the reservoir are a serious contamination concern. They may introduce fibers, detergent residues, dirt, water, and polar contaminants. They can also become nucleation sites for sludge and varnish.

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4. Materials with Moderate Varnish Attraction

Stainless steel

Stainless steel is usually better than carbon steel, but it is not immune.

Its passive chromium oxide layer is polar. This oxide layer can adsorb polar oxidation products, especially if the surface is rough, scratched, heat-tinted, chemically attacked, or contaminated.

Typical oxide:

Cr₂O₃ dominant
Fe oxides
Ni oxides

Stainless steel is less catalytic than copper and less rust-active than carbon steel, but in servo valves, fine strainers, and small clearances, varnish can still be a major problem.

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Aluminum and aluminum alloys

Aluminum forms a strong oxide layer:

Al₂O₃

Aluminum oxide is highly polar and has high surface energy. It can adsorb polar species strongly.

However, aluminum is less common in direct high-temperature turbine oil wetted critical surfaces compared with steel, Babbitt, copper alloys, and stainless steel.

Where aluminum alloy parts exist, surface oxide polarity can make them varnish-attractive.

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Zinc-containing surfaces or galvanized parts

Zinc is generally not preferred in critical turbine oil systems. Zinc oxide and zinc soaps can interact with acidic degradation products.

Potential compounds:

ZnO
zinc carboxylates
zinc-containing sludge species

Zinc surfaces or zinc contamination may contribute to ash-type or soap-like deposits.

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5. Lower Varnish Attraction Materials

Fluoropolymers such as PTFE

PTFE has very low surface energy and is non-polar.

It generally has low attraction for polar varnish products compared with oxidized metals.

However, PTFE parts can still be contaminated mechanically if sticky varnish, sludge, or particles are circulating.

PTFE is less likely to chemically attract varnish, but not impossible to foul.

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Some elastomers

Elastomers are complex. They may not always “attract” varnish like metal oxides, but they can absorb oil degradation products.

Common elastomers:

NBR
FKM / Viton
EPDM
HNBR

FKM generally has better thermal and chemical resistance than NBR. NBR can harden, swell, or absorb polar degradation species depending on oil chemistry and temperature.

Deposits around seals may be due to:

thermal stress,
oil leakage and oxidation at air interface,
seal degradation,
additive interaction,
surface temperature,
external contamination.

So elastomers are not always the strongest varnish attractors chemically, but seal areas can become deposit-prone because of heat, oxygen exposure, and leakage films.

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6. Why Hot Areas Attract More Varnish

Temperature has two opposite effects.

At high oil temperature, varnish precursors may remain dissolved better.

But high temperature also accelerates oxidation and thermal degradation.

At hot metal surfaces, such as bearings, thrust pads, seals, and control oil components, the local oil film may experience:

thin-film oxidation,
thermal stress,
additive depletion,
micro-coking,
polymerization,
local acidity increase,
boundary lubrication.

So hot surfaces may generate and attract deposits at the same time.

This is why varnish on a bearing surface is not only a chemistry issue. It is also a tribology issue.

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7. Why Cooler Areas Also Attract Varnish

This sounds contradictory, but it is very important.

Hot oil can carry soluble degradation products. When it reaches a cooler surface, the solubility of these polar oxidation products decreases.

Then they can precipitate or adsorb.

Therefore varnish can deposit both in:

hot zones, because degradation is generated there, and
cool zones, because solubility drops there.

This is why coolers, reservoirs, return lines, and low-temperature dead legs can collect sludge/varnish even when the main machine is hot.

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8. Surface Roughness and Metallurgical Condition

A polished surface and a rough oxidized surface do not behave the same.

Roughness increases:

surface area,
mechanical anchoring,
boundary layer retention,
local low-flow zones,
adsorption sites.

Metallurgical damage also matters.

Examples:

fretting,
corrosion pits,
wipe marks,
fatigue cracks,
smearing,
erosion,
cavitation marks,
electrical discharge marks.

These surface defects act as deposit anchors.

A varnish molecule may first adsorb chemically, then more molecules attach physically, and finally a visible layer grows.

So varnish formation is often a sequence:

molecular adsorption → thin polar film → sticky soft deposit → oxidized resin layer → hard varnish/sludge layer.

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9. Why Oxide Layers Are So Important

Most metallic surfaces in oil are not pure metal. They are oxide-covered metals.

This oxide layer controls surface polarity.

Examples:

Material	Surface oxide	Varnish relevance
Carbon steel	FeO, Fe₂O₃, Fe₃O₄, FeOOH	Strong polar adsorption, rust activity
Cast iron	Iron oxides + graphite zones	Rough, reactive, deposit-prone
Copper	Cu₂O, CuO	Catalyzes oxidation, polar surface
Brass/bronze	Cu/Zn/Sn oxides	Polar and chemically active
Tin Babbitt	SnO, SnO₂	Polar, soft bearing surface
Lead Babbitt	PbO	Reactive, deposit-sensitive
Stainless steel	Cr₂O₃	Stable but polar passive layer
Aluminum	Al₂O₃	Very polar, high surface energy
Zinc	ZnO	Can react with acids, form soaps/sludge

In varnish chemistry, the oxide layer is often more important than the base metal itself.

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10. Why Bearings Are High-Risk Varnish Locations

Journal and thrust bearings combine nearly all varnish-promoting factors:

high temperature,
thin oil film,
high load,
boundary/mixed lubrication during start-stop,
Babbitt metallurgy,
soft reactive surface,
possible copper/tin/lead chemistry,
oil residence in grooves,
surface wiping/polishing,
local oxidation,
low clearance sensitivity.

Varnish on bearing surfaces can cause:

higher bearing temperature,
reduced heat transfer,
disturbed oil film,
reduced clearance,
localized wiping,
increased friction,
false indication of alignment or loading problems,
unstable vibration behavior.

A varnished bearing may look like a bearing problem, but the root cause can be oil chemistry.

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11. Why Servo Valves Are Even More Sensitive

Servo valves may not collect the largest mass of varnish, but they are the most functionally sensitive.

A few microns of deposit can cause:

stiction,
slow response,
hysteresis,
positioning error,
trip events,
unstable control,
increased actuator hunting.

The surface may be stainless steel or hardened steel, but the problem is clearance.

In a reservoir, 5 microns of deposit means almost nothing.

In a servo valve, 5 microns may be a failure.

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12. Why Filters and Filter Housings Can Collect Varnish

Filter systems can be varnish hot spots because of:

electrostatic charging,
high surface area,
polar filter media,
flow restriction,
pressure drop,
particle concentration,
resinous oxidation product capture,
temperature drop.

Some filter media are cellulose-based and polar. Cellulose has hydroxyl groups, which can hydrogen-bond with polar varnish precursors.

So filter media may capture some insoluble varnish particles, but this does not mean they remove soluble varnish precursors effectively.

This is a very important distinction:

Mechanical filtration can remove particles and some insoluble soft contaminants.
It cannot reliably remove dissolved polar varnish precursors from hot oil.

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13. Root Cause Logic: Why the Same Oil Deposits More on One Material Than Another

The deposition risk depends on five combined factors:

1. Oil chemistry

Oxidation level, antioxidant depletion, TAN/SAN, MPC, RULER, RPVOT, water, metals, contamination.

2. Surface chemistry

Oxide type, polarity, catalytic activity, coating chemistry, elastomer compatibility.

3. Surface condition

Roughness, corrosion, scratches, fretting, wiping, deposits from previous oil.

4. Operating condition

Temperature, flow velocity, dead zones, pressure drop, aeration, electrostatic charge.

5. Geometry

Clearance, residence time, boundary layer, stagnant pockets, cooler areas, hot spots.

This is why varnish is a system problem, not only an oil problem.

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14. Practical Ranking in a Turbomachinery Lubrication System

From Khash field experience, I would rank varnish-prone wetted surfaces approximately like this:

Very high risk

Babbitt journal bearing surfaces
Thrust bearing pads
Servo valve spools and sleeves
Control valve internals
Copper alloy cooler tubes
Oxidized carbon steel reservoir areas
Dead-zone reservoir floors and baffles
Fine strainers and filter housings
Small-bore control oil lines

High risk

Carbon steel piping with oxide layer
Cast iron housings
Bronze/brass bushings and fittings
Stainless steel valve blocks in low-clearance systems
Cooler outlet zones
Return lines with temperature drop
Old epoxy-coated reservoir internals

Moderate risk

Clean stainless steel piping
Aluminum alloy components
Seal areas
Instrumentation tubing
Non-critical wetted metallic surfaces

Lower risk

PTFE surfaces
Very smooth inert polymer surfaces
Clean low-energy synthetic polymer surfaces

But in real life, operating condition can override material ranking. A stainless steel servo valve may fail before a carbon steel reservoir because its clearance is much smaller.

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15. Important Point: Varnish Attraction Is Not Only “Sticky Dirt”

Varnish is not just dirt sticking to metal.

It is a surface-chemistry process:

polar molecule formation,
molecular adsorption,
surface bonding,
deposit growth,
oxidative hardening,
thermal aging,
particle embedding.

The first layer may be invisible. But once the first polar film forms, it becomes easier for more polar material to attach. This is why varnish can accelerate once surfaces become conditioned.

Old varnished systems often have “memory.” Even after changing oil, the old deposits and polar surfaces continue to influence the new oil.

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16. How to Investigate Which Material Is Attracting Varnish

For a professional RCA, do not rely only on visual inspection.

Use:

MPC with patch photo
Not only MPC number. The patch color and Delta L/a/b give clues.

RULER / LSV
To check antioxidant depletion.

TAN by ASTM D664
To monitor acidic degradation.

RPVOT
To understand oxidation stability reserve.

FTIR
To identify oxidation, nitration, additive changes, and contamination patterns.

SEM/EDS on deposits
To identify inorganic elements such as Fe, Cu, Sn, Pb, Si, Zn, Ca, P, S.

Ferrography / analytical microscopy
To separate wear debris from soft varnish/sludge.

Deposit solubility test
To understand whether the deposit is organic varnish, carbonaceous material, inorganic contamination, or mixed material.

Surface inspection
Check roughness, corrosion, wipe marks, heat tint, bearing overlay condition, and oxide condition.

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17. What SEM/EDS Can Tell You

If varnish is only organic oxidation product, EDS may show mostly carbon and oxygen, but EDS is weak for light organic chemistry.

If EDS shows metals, it can indicate:

Fe: rust, steel wear, iron oxide contamination
Cu: copper cooler tubes, bronze/brass, bearing alloy, catalytic copper compounds
Sn: Babbitt bearing material
Pb: lead-based Babbitt
Sb: Babbitt alloy element
Zn: galvanized material, additive/deposit interaction
Si: dust, sealant, silicone, sand, antifoam-related contamination
Ca/Mg: detergent contamination, grease contamination, external contamination
P/S: additive degradation, antiwear chemistry, phosphate-type material, contamination

A real varnish deposit is often mixed:

organic polar resin + metal oxides + fine wear debris + dust + additive ash + carbonaceous material.

This mixed chemistry is why varnish can be sticky, abrasive, insulating, and difficult to remove.

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18. Field Interpretation of some of Khash Photos

From the photos, I would not make a final diagnosis only visually, but technically I would immediately suspect:

reservoir contamination and poor housekeeping risk,
possible sludge/varnish collection in low-flow reservoir areas,
bearing surface staining consistent with oil degradation/thermal film/deposit interaction,
possible polar deposit attraction on Babbitt/metal surfaces,
need for deposit analysis instead of visual judgment only.

The bearing surface area deserves careful inspection because Babbitt metallurgy plus hot thin-film lubrication is one of the most varnish-sensitive locations in turbomachinery.

The reservoir photo also raises a serious concern: cloth/rags inside an oil reservoir can become contamination sources and nucleation points for sludge and varnish. In a critical turbomachinery lube system, this is unacceptable from a lubrication cleanliness and reliability perspective.

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19. Final Technical Summary

The materials that attract more varnish in turbomachinery lubrication systems are generally those with:

high surface polarity,
oxidized metal layers,
catalytic metal chemistry,
high surface energy,
micro-roughness,
small clearances,
hot operating surfaces,
cool precipitation zones,
low-flow or stagnant geometry.

The strongest varnish-attracting or varnish-sensitive materials are usually:

oxidized carbon steel, cast iron, copper alloys, Babbitt bearing metals, stainless/tool steel servo valve surfaces, cooler tubes, aged epoxy coatings, and polar filter media.

The most important chemistry is the interaction between:

polar oil degradation products
and
polar metal oxide / hydroxide surfaces.

In practical turbomachinery reliability terms:

Varnish is not only an oil cleanliness issue.
It is an oil chemistry + surface chemistry + metallurgy + temperature + flow regime issue.

That is why one machine can show heavy varnish while another machine with the same oil brand, same turbine model, and similar maintenance routine may remain clean. The difference is often hidden in the surface condition, metallurgy, thermal stress, reservoir design, flow pattern, and history of oil degradation.