In my last several columns, I discussed some suggestions on oil temperature and oil change intervals. I continue to receive questions on which oil to use and the compatibility between oils.
First off, I am talking about certified aircraft engines like Lycoming, Continental, Pratt & Whitney, CW, etc. In these engines, you should NEVER use automotive or heavy duty truck oils. You can only use oil that meets the Mil-L 22851/SAE 1899 AD or Mil-L-6082/SAE 1966 mineral oil specification.
Mineral oil is used primarily for break-in of new engines or cylinders. Always check with your engine rebuilder or engine manufacturer as to which oil to use in your engine during break-in.
AD or ashless dispersant oils are what pilots normally use for the rest of the engine’s life.
The two oils are compatible and can be interchanged, except for one example.
If you buy a used aircraft, it is important to determine what oil was used previously. Occasionally, you will find an engine that has been flown its entire life on just mineral oil. If the engine is fairly high time — say above 250 hours — then you are probably better off staying with just mineral oil.
If you do decide to change to AD oil, you will need to monitor your engine closely and check the screens often because AD oil can loosen up carbon deposits and plug your filter and oil screens.
Other than that one example, you can switch back and forth with no problem.
For example, if you have a high time engine run on AD oil, and need to replace a cylinder, we have never had a problem using mineral oil in the engine for one or two oil changes and then going back to AD oil.
So what grade oil should you use in your aircraft?
The first step is to look at the appropriate service bulletin for your engine model.
For example, many of the small Continental engines do not recommend using a grade 100 or 50W oil under normal operation. They recommend using 80 grade oil under normal operating conditions.
The big radials normally use a 120 grade oil for most operations.
So what viscosity grade would I recommend?
For most flat engines, I recommend using single grade oil for the warmer weather and a multi-grade oil during the coldest part of the year. The multi-grade oils provide faster cranking, better chance of starting, and the oil gets to critical bearing surfaces quicker after start.
There is an old belief that switching from single grade to multi-grade or switching brands can cause problems in your engine or cause your engine to use oil. This is not completely true. All qualified oils have been tested for compatibility with all other approved brands of oil, so no problem here.
When you switch from a single grade to a multi-grade oil, you may notice some difference in oil consumption. If your engine leaks oil or loses some oil past the intake valve guides, then because of the improved flow characteristic, you can see an increase in oil consumption.
Likewise, if your engine is seeing most of your oil consumption going past the oil rings, then you may see a 30% to 50% reduction in oil consumption when you switch from single grade to a multi-grade. But this is not harming your engine and the consumption will return if you go back to a single grade.
I know that I have shared this information in previous columns, but I like to repeat the basic lesson every so often to reinforce its importance.