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aviation resources
Compatibility: Is it a myth or reality?
Compatibility: Is it a myth or reality?
Ben Visser
11/26/2004
Can you mix various grades and brands of oils?
Many people have questions about this, stemming from an old tradition in automobiles that once you start with one
brand of oil you should never change. We have all heard about some guy who bought a car that had been running brand
X. After he purchased the car, he switched it over to his favorite brand Y oil, and the engine started using oil -
or it blew up within a few miles. The same stories have been in aviation for as long as I have been in the business.
These automotive traditions began in the 1950s when detergent oils first appeared on the market. There were some
very different additives being marketed and some of them were not compatible with other brands. In aviation, the
primary basis for the tradition comes from when ashless dispersant (AD) oils were first introduced in 1958. Because
of the superior performance of the AD oils, many airlines and private pilots switched their high time engines over
to the new product. In many of these cases, the AD oil loosened carbon deposits, which were washed into the oil
filter or oil screens. In severe cases, the excessive amount of carbon plugged the oil screens and starved the oil
flow to the engine.
What about today? All brands of aviation piston engine oils must be approved by manufacturers and FAA under the
Military Specification Mil-l-22851 and the SAE J-1899 specification. This spec is for all certified piston engines,
including those made by Continental, Lycoming, Pratt & Whitney and others.
To be approved, a product must pass a compatibility test with all other approved products. This means that you can
go from Brand X to Brand Y and back again with no significant problems.
The same is true of different grades of oil. For example, many pilots use a single grade during the summer months
and then switch over to a multi-grade oil in the winter. Here again, there should be no significant problems.
However, you may note some small changes. For example, multi-grade oils tend to leak better than single-grade oils,
so if you put a multi-grade oil in your engine, you may notice a little more oil consumption if you have a leaker.
Or you may notice a significant reduction in oil consumption with the use of a multi-grade oil — it just depends on
your engine type and condition. This change in oil consumption is a result of the physical properties of the oils
and does not indicate any negative change to your engine.
You can even mix grades of oil together. If you have a multi-grade AD oil in your aircraft, and the only oil
available is a grade 100 AD oil, you can add it with no significant problems. The grade 100 oil will cause a small
increase in the low temperature flow characteristics for cold temperature startability. But under normal cruise
conditions, the mixture will perform the same as straight multi-grade.
If you replace a cylinder on a mid-time engine, you may want to run a mineral oil for a few hours to break in the
new cylinder. In this case you can switch from AD to mineral and back to AD oil with no problems. The only caution
is that if you have a high time engine that has been run on only mineral oil, I would stay with mineral oil until
the engine is overhauled.
The good news is that mixing brands and grades of approved oil is not critical.
The bad news is that oil temperature and change intervals are.
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 visser@generalaviationnews.com.
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