Are aviation lubricants compatible?

In the mid-1950s, I went along with my father to my uncle’s service station to buy some oil. While we were there, the man who owned a service station in the neighboring town came in to buy some oil.

My uncle, who knew the man well, asked him if he came in to buy the good stuff as opposed to the brand that he sold. After a few laughs, the man explained that he had just purchased a used vehicle and the previous owner had always used my uncle’s brand of oil. He claimed that if you changed brands, it could ruin your engine.

When I went to work for Shell Oil, I remembered that story, so I started to look into whether it was true.

The problem began in the late 1940s and 1950s when oil companies introduced detergent oils into the market. Prior to this they had been selling straight mineral oil, so when people switched to detergent oils, the oil cleaned up the carbon in the engine — and plugged small passages in the engine. In addition, there was a wide variety of different chemistries in the different brands that did not mix well with others.

In the late 1960s, the specifications changed. One of those changes was a compatibility requirement. Now people can mix almost any qualified oil with other brands and grades.

The specification keeps changing every few years, but the new oils should work in all older engines.

The exception here is that new engines are almost all roller lifter and catalytic converter equipped, so the latest spec calls for reduced zinc levels. If these oils are used in old flat tappet equipped engines, especially just after overhaul, the lifters tend to fail. So always look for an oil with a higher zinc level for older cars.

But What About Aviation?

In aviation, there are just mineral oil and ashless dispersant oils, which were introduced in 1958. You can mix any of the qualified brands and grades of oil.

The only qualifier I would add is not to switch a high-time engine that has been run on only mineral oil over to an AD oil.

You can also change from single grade to multigrade oil. The only concern here is that the oil consumption may vary depending on how your engine is consuming the oil.

What about grease compatibility?

Most non-aviation greases usually mix well with others, but you may need to separate them by usage.

For example, I have a separate grease gun that I only use for lubricating snow mobiles because it has very good low-temperature properties. I also have separate equipment for my boat trailer because of the improved water wash out characteristics. Always try to match the grease to the end usage.

In aviation, most greases do not mix well with others. For example, Aeroshell Grease 14, when mixed or applied on top of most other aviation greases, will form a soup-like mixture that will leak everywhere.

Also, if you apply Grease 5 on top of Grease 6, you will have some very stiff controls, especially in cold weather.

Likewise, if you top off your wheel bearing that was designed for Grease 5 with full synthetic Grease 7, they will mix, but the mixture will probably eat the seals, letting dirt in while the grease goes out.

The bottom line is the non-aviation world is now fairly forgiving on compatibility. In the aviation world you will be OK if you use a qualified engine oil. But for grease, the only safe bet is to use the grease that is recommended for each lubrication point.