Lubrication science
Synthetic lubricant base stocks: PAO, esters, PAG and more
Quick answer
What each synthetic family is good at, where it can cause trouble and what not to mix.
Why synthetics exist
Synthetic base stocks are built molecule by molecule to a design. Compared with mineral oil they hold their viscosity better, tolerate higher and lower temperatures and often last longer. They cost more, so they are used where heat, cold, long drains or hard duty justify it.
The main families
| Base stock | Strengths | Watch out for | Typical use |
|---|---|---|---|
| Polyalphaolefin (PAO) | High viscosity index, excellent low-temperature flow, very good oxidation stability, mixes with mineral oil | Poor solvency for some additives, may shrink some seals unless an ester is blended in | Gear, bearing, compressor and circulating oils, greases |
| Polyalkylene glycol (PAG) | Very high film strength and low friction, clean running, high viscosity index | Does not mix with mineral oil or PAO. Attacks some paints and seals. Takes up water | Worm gears, some compressors, special systems |
| Diesters and polyol esters | Good lubricity, high temperature stability, biodegradable options | Can swell seals and attack some paints. May hydrolyse with water | Turbine, aviation, refrigeration and biodegradable fluids |
| Alkylated naphthalene | Good thermal and hydrolytic stability, good additive solvency | Used mostly as a blend component | Blended with PAO in industrial oils |
| Silicone | Very wide temperature range, low volatility | Poor lubricity for steel on steel | Damping and special-purpose fluids |
Changing from mineral to synthetic
- Check seal and paint compatibility with the supplier before changing.
- Never mix PAG with mineral oil or PAO. Drain and flush.
- Esters and PAO usually mix with mineral oil, but the blend loses some of the benefit.
- Treat a change of base stock like a change of lubricant: update labels, records and the oil analysis baseline.
Cost against benefit
A synthetic oil is worth the price where it removes a failure, such as a gearbox that overheats, a cold-start problem or a drain interval that is too short. It is not worth it where a mineral oil already runs cool and clean.
Takeaway. Pick the synthetic family for the problem you are solving, and treat any change of base stock as a controlled change with flushing and a new oil analysis baseline.
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