Synthetic oil: the myth and the reality

Thirty years ago, companies started selling synthetic oils and lubricants for automotive and other reciprocating engine uses. They now comprise a moderately large segment of the market, and are available form specialist vendors-Amsoil, Klotz, Bel-Ray-and the biggies alike.

More nonsense has been written about these products than accurate information and before blindly paying for them you should know a little about their history, their advantages and their disadvantages.

Synthetic oils were first widely used in aircraft turbine engines. Because of the chemistry available then, those oils were esters of a non-compatible type to petroleum oils and to the rubber seals and hoses they used. The aircraft industry was able to completely switch over entire systems to those materials, just as they were able to switch hydraulic systems-every seal, bladder, O-ring and flexible hose-from petroleum to phosphate ester (Skydrol) based fluids.

Those synthetic oils were used because they could handle the temperature extremes of turbine engine operation better than regular oils, which would gel and freeze at the cold temperatures in high altitude subsonic flight and coke inside the engine at high temperature low altitude and ground operation, supersonic cruise regimes, and-the big problem-on shutdown when the bearings would "heat soak". But for all the zoomy high tech aura of jet engines, they did not abuse the oil in ways piston engines do. They don't foul the oil with combustion products because the bearings are not in contact with combustion products. There are no piston rings on bore walls to shear it, no cam lobes to provide extreme pressure to the lubricating film (nor to need that level of EP adhesion), no blowby and soot and raw fuel. There's very little air in the system, even.

Putting synthetic oils in piston engines meant, first, using a different chemistry compatible, more or less, with existing oils. It did provide much better oil consistency at temperature extremes, which was a huge benefit for Arctic operations, and in racing, where teams could run smaller oil coolers. And of course, being expensive and associated with aircraft, it had a lot of snob appeal and cachet. That was the real motivation of the mass purveyors of the stuff, such as Al Amatuzzio of AMSoil.

Synthetic oils, today, have improved a little for piston engine use since then, but not a lot. They still have the same advantages and the same disadvantages.

Disadvantages?

Yes. For one thing, they are only partly compatible with petroleum oils and, more importantly, with seals and gaskets that have been used extensively with synthetic oils. There is no doubt that when switching engines over the likelihood of seal and gasket failure is quite high. The longer the engine has been operated since overhaul, the higher the risk. Anyone who says otherwise is not being straight with you.

Another problem is that synthetics do a lousy job of holding contaminants in suspension. If you doubt that, ask the synthetic oil people why they don't sell an approved AD aircraft piston engine oil. They did. It worked fine in the small Lycoming and Continentals operated on 80/87 or mogas-but the bigger flat engines suffered damage because the large amounts of tetraethyl lead in 100 octane avgas formed a paste and plugged oil passages. I know A&P mechanics who use synthetic car oils in their A and C series Continentals they fly on mogas. But they'd be the first to say they don't recommend anyone else do so.

Extended drain intervals? Yes, you can, if you use a system with a bypass filter in addition to the stock full flow one, and have the oil tested. It's not cost effective for private users to do that-the oil analysis is more than an oil change.

Bypass filters, in conjunction with the stock one, do do a lot of good. But you don't need synthetic oils for that. Several are available, but my favorite-IF you change it regularly-is the time tested toilet paper Franz filter.