Why Old Group I Turbine Oils Were “More Polar” and Group II / III Are “Less Polar” — and Why This Changes Varnish Behaviour

When a Turbine engineer asked me above , I replied:

First, one correction:

Group I turbine oil is not truly “polar” like water or phosphate ester fluid.
It is still a hydrocarbon oil and mainly non-polar. But compared with modern Group II and Group III base oils, Group I contains more naturally polar molecules.

That small difference has a very large effect on oxidation-product solubility, varnish suspension, deposit formation, MPC behaviour, and additive response.

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1. Why Group I Oils Were More Polar

Group I base oils are solvent-refined

Old turbine oils were commonly based on API Group I mineral oils, produced mainly by solvent refining.

This process removed some unwanted compounds but left behind a meaningful amount of:

• Aromatics
• Naphthenic structures
• Sulfur-containing molecules
• Nitrogen-containing molecules
• Other heteroatomic compounds
• Natural polar species

These molecules gave Group I base oils:

Higher solvency
Higher natural polarity
Better ability to keep degradation products dissolved or dispersed

In simple words:

Group I oil had a better “natural cleaning and dissolving character.”

It was not necessarily cleaner or more oxidation-stable, but it could hold many oxidation by-products in solution for longer.

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2. Why Group II and Group III Oils Are Less Polar

Group II and Group III base oils are produced by more severe hydroprocessing and hydrocracking.

This removes most of the:

• Aromatics
• Sulfur compounds
• Nitrogen compounds
• Unsaturated molecules
• Naturally polar compounds

As a result, Group II and Group III oils have:

• Higher saturates
• Lower sulfur
• Better color
• Better oxidation stability
• Better thermal stability
• Higher VI, especially Group III
• Much lower natural solvency

They are chemically “cleaner,” but also less capable of dissolving polar degradation products.

This is the key paradox:

Group II and III oils resist oxidation better, but when oxidation products are formed, they may have less ability to keep them dissolved.

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3. The Critical Word: Solvency

Varnish behaviour is not only about how fast oil oxidizes.

It is also about:

How much degradation material the oil can hold before it drops out.

This is called solvency.

Think of oil like tea.

Group I oil is like stronger tea that can hold more dissolved material.

Group II / III oil is like very clean water with less tolerance for contamination. It may look beautiful, clear, and bright, but when degradation products form, they may separate more easily.

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4. Oxidation Products Are More Polar Than the Base Oil

When turbine oil oxidizes, it forms degradation products such as:

• Organic acids
• Aldehydes
• Ketones
• Esters
• Alcohols
• Peroxides
• Resinous oxidation products
• Polymerized soft contaminants
• Sludge and varnish precursors

Many of these are polar or semi-polar molecules.

Now compare them with the base oils:

Property	Group I	Group II / III
Natural polarity	Higher	Lower
Aromatic content	Higher	Very low
Solvency	Better	Poorer
Oxidation stability	Lower	Higher
Ability to hold varnish precursors	Better	Lower
Tendency for sudden deposit drop-out	Lower	Higher

So in Group II and III oils, oxidation products can reach their solubility limit sooner.

That means they may come out of solution and form deposits.

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5. How This Affects Varnish Formation

In old Group I turbine oils

Group I oils oxidized faster, but their higher solvency allowed them to keep more oxidation products dissolved.

Typical behaviour:

• Oil gradually became darker
• TAN might slowly increase
• Sludge could form over time
• Degradation was often more visible
• Color change was more obvious
• Operators had earlier visual warning
• Deposits could be more sludge-like

In many cases, Group I oil gave the maintenance team “visible warning signs.”

The oil became darker, dirtier, and more obviously aged.

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In modern Group II / III turbine oils

Group II and III oils can remain visually clean for a long time.

But because they have lower solvency, polar oxidation products may separate earlier once the oil becomes saturated.

Typical behaviour:

• Oil may still look clear and bright
• ASTM D1500 color may look acceptable
• TAN may remain low
• Particle count may look acceptable
• But MPC may increase
• Sticky deposits may form on cool surfaces
• Servo valves, bearings, hydraulic actuators, and trip systems may become affected

This is why modern oils can be dangerous from a diagnostic point of view:

The oil can look healthy while varnish potential is already high.

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6. Why Group II / III Oils Can Have “Clean Oil, Dirty Machine” Behaviour

This is one of the most important points.

Modern Group II / III turbine oils are often:

• Bright
• Clear
• Low in sulfur
• Low in aromatics
• Low in natural color
• Low in natural solvency

So the oil sample may look excellent.

But the machine surfaces may tell another story.

Varnish may deposit on:

• Journal bearing surfaces
• Thrust bearing pads
• Servo valves
• Hydraulic control valves
• Seal oil systems
• Cooler surfaces
• Reservoir walls
• Small oil passages
• Trip and control systems

This creates the classic problem:

Beautiful oil sample, ugly internal surfaces.

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7. Temperature Makes the Problem Worse

Varnish solubility is highly temperature-dependent.

At higher temperature, oil can hold more soluble varnish precursors.

At lower temperature, oil loses solvency and polar degradation products may precipitate.

This is why deposits often form in:

• Coolers
• Reservoirs
• Low-flow zones
• Standby systems
• Control oil lines
• Servo valves
• Cooler return areas
• Machines after shutdown

Important mechanism:

Hot oil dissolves more varnish precursor.
Cold oil releases more varnish precursor.

So when a hot turbine shuts down, oil temperature drops, and soluble degradation products can become insoluble and deposit on sensitive surfaces.

This is especially critical in Group II / III oils because their solvency reserve is lower.

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8. Why Varnish Often Appears After Oil Change to Group II

Many plants changed from Group I to Group II turbine oil expecting better performance.

Chemically, Group II oils often provide better oxidation resistance.

But in dirty or aged systems, the changeover can trigger varnish problems.

Why?

Because old Group I oil may have been holding old degradation products in solution. When replaced with Group II oil, the new oil may not dissolve the same amount of aged polar contamination.

Possible result:

• Old deposits are disturbed
• Old soluble contaminants lose solubility
• Residual oxidation products become unstable
• Varnish precursors precipitate
• MPC rises
• Valves become sticky
• filters may plug faster
• deposits appear on cooler surfaces and hydraulic components

This is why oil conversion without proper system cleaning can create problems.

Not because Group II is “bad,” but because:

Group II has different chemistry, different solvency, and different varnish behaviour.

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9. Additive Solubility Is Also Affected

Modern turbine oils rely heavily on additives, including:

• Antioxidants
• Rust inhibitors
• Demulsifiers
• Anti-foam agents
• Metal passivators
• Sometimes yellow metal inhibitors

Some additives are polar molecules.

In Group II and Group III oils, because of lower natural solvency, additive compatibility and solubility become more formulation-sensitive.

This affects:

• Additive response
• Additive depletion pattern
• RULER interpretation
• Deposit tendency
• Filterability
• Water separation
• Foam and air release
• Compatibility after top-up or mixing

A well-formulated Group II turbine oil can perform extremely well, but it depends more heavily on good additive chemistry.

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10. MPC Behaviour: Why Group II / III Oils Need More Attention

MPC, ASTM D7843, is one of the most important tests for modern turbine oils because it measures varnish potential more directly than simple color, TAN, or particle count.

In Group II / III oils, MPC can increase even when:

• TAN is still normal
• RULER is still acceptable
• Particle count is acceptable
• Water is low
• Oil color is light
• Viscosity is stable

This happens because varnish precursors can be soluble, submicron, or soft polar degradation products not captured by ordinary particle counting.

So for modern Group II / III turbine oils:

MPC is not optional. It is essential.

And the patch photo is also important, because patch color and character can reveal different degradation mechanisms.

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11. Practical Example

Imagine two turbine oils operating under the same thermal stress.

Group I oil

Oxidation products form faster, but oil holds them longer.

You may see:

• Darker oil
• Gradual TAN increase
• More visible aging
• Sludge tendency
• Slower sudden varnish precipitation

Group II oil

Oxidation products form slower, but once formed, the oil has lower tolerance to hold them.

You may see:

• Clean-looking oil
• Low TAN
• Good color
• Sudden MPC increase
• Servo valve sticking
• Bearing surface staining
• Deposits after shutdown or cooling

This is why Group II oils can sometimes produce a surprise failure mode:

The oil report looks normal, but the control system starts misbehaving.

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12. The Key Reliability Message

For old Group I oils, traditional oil analysis such as color, TAN, viscosity, water, and particle count often gave useful warning.

For modern Group II and Group III oils, traditional tests are not enough.

You need:

• MPC
• RULER
• RPVOT
• FTIR oxidation trending
• TAN by ASTM D664
• Particle count
• Water by Karl Fischer
• Patch membrane inspection
• Visual inspection of deposits
• Servo valve performance history
• Bearing temperature trends
• Filter differential pressure trends

Because modern oil failure is often not simply “oil dirty.”

It is more like:

Chemistry instability + low solvency + polar oxidation products + temperature cycling = varnish risk.

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13. Simple Khash Summary

Old Group I turbine oils were like an older engineer with dirty hands but strong field experience.

They were not chemically perfect, but they could tolerate and hold more contamination.

Modern Group II and III oils are like a clean, polished, high-performance engineer.

Very efficient, very stable, very clean — but less forgiving when polar degradation products start accumulating.

So the varnish behaviour changed:

Group I: more oxidation, better solvency, more visible aging.
Group II / III: less oxidation, poorer solvency, more hidden varnish risk.

Final sentence:

Modern Group II and III turbine oils do not always fail by becoming dirty. They often fail by becoming chemically saturated with invisible polar degradation products that later become visible as varnish on machine surfaces.