Stump the expert

Own an antique engine?

Own an antique engine?

So does Paul Cullman, who has an aircraft with a Salmson 9AD engine built in 1928.

The manual for this engine states: “”For summer use mineral oil Mobiloil Aero H and for winter Mobiloil W.”" The manual also states that if the above lubricants are unavailable, in summer use “”B”" and in winter, Mobiloil “”BB.”"

Cullman called Exxon Mobil and could not find out what grade of oil should be used in his engine. The gentleman who overhauled his engine recommended Phillips 20W-50.

If we are playing stump the expert, Cullman would win the prize. I have looked through my library and cannot find any reference to these oils or grades. I even called some really old people, and no one knew for sure what the definitive answer was. So I am going to give my best guess based on my knowledge and that of several others.

The main problem that can be encountered is that of switching from straight mineral oil to an ashless dispersant (AD) oil in an old engine. The AD oil can clean up the engine and plug oil screens and other small passages, which can lead to serious problems.

However, since the engine is being overhauled, it should already be clean, so an AD oil will work well.

Also, if re-chrome cylinders are used, it may be a good idea to break-in the engine with a mineral oil to ensure proper ring seating. If the rebuilder does not specify a mineral oil for break-in, than the use of an AD oil like Phillips 20W-50 should work well. Phillips also markets a 20W-50 mineral oil that works well for break-in.

In antique aircraft, if the engine was originally designed for mineral oil, you can use either a Mil L 6082 mineral oil or, if it is clean, a Mil L 22851 AD oil.

If the engine was originally designed for castor bean oil, that is another story. Here you would be better served by staying with castor oil in most cases.

Back to the original question of what weight oil was Mobiloil H and W. I still have not found a definite answer to that question. Back then oils were not classified by SAE or aviation viscosity grades like we have today. Oils were either light, medium or heavy. Best guess is that the Mobiloil W was about an SAE 30 weight or grade 65 oil. The Mobiloil H was probably in the SAE 40-50 weight range (aviation grade 80-100). The Mobiloil B and BB were probably similar in viscosity, but contained a pour point additive and were marketed for non-aviation use.

The bottom line here is that, when buying an aircraft, always try to find out what type of oil was used previously. If the plane was operated on straight mineral oil, I strongly recommend that you stay with that oil, at least until overhaul. If it was operated on an AD oil then you can use either a mineral oil or an AD oil.

The brand of oil is not that critical since all approved aircraft engine oils are tested for compatibility with other brands.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at

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