The two most common questions that I still get are: 1. What oil should I use in my aircraft engine? And 2. Are different brands and grades of oils compatible?
In this column I will limit the discussion to certified engines, such as Continental, Lycoming, radials and older certified aircraft engines. For these engines there are four different choices.
First is straight mineral oil that meets the SAE J-1966 and Mil-L-6082E specification. The second is single grade ashless dispersant or AD oils that meet the SAE J-1899 and MIL-L-22851 D specification. The third type are multi-grade oils that meet the same SAE J-1899 and Mil-L-22851D specification. The last type of oils meet the same spec as the AD and multi-grade oils, but also contain the Lycoming LW 16702 additive or equivalent and are referred to as anti-wear oils.
So are these oils all compatible with the other choices? Well, usually yes, but there are some notable exceptions.
The straight mineral oils are usually used only for break-in of new engines. To determine which oil to use when breaking in your new engine, always check with your manufacturer or rebuilder. (I also discussed this in one of my previous columns, which ran in the Nov. 23, 2012, issue. It’s also available at GeneralAviationNews.com.)
Some people will use straight mineral oil for the entire life of the engine, which is legal. But this can lead to problems if the aircraft is sold and the new owners change to an AD oil. The AD oil can loosen up some carbon deposits, which can plug screens and filters, causing oil flow problems and even engine failure. Problems may only occur in, say, 20% of the change overs, but that is a significant level.
The big question is what is the limit in hours after which it is not advisable to switch to an AD oil? There is no hard and fast limit here because it depends on how the plane was flown and maintained. My best guess would be to not change over an aircraft after 200 to 300 hours — and even then monitor the oil screens and filters very carefully.
If you lose a cylinder mid TBO, you can switch from AD oil back to a mineral oil for a few hours, and then switch back to AD oil. The problem occurs from extended operation on the mineral oil, so always determine which type of oil was used when buying a used aircraft.
Another note is that many rebuilders do not recommend using the anti-wear type oil during break-in, except for engines like the Lycoming O-320-H, which must use the additive or one of these oils.
Other than these exceptions, all approved oils are compatible. You can switch brands and grades without fear of harming your engine. You may see some change in oil consumption when switching from a single grade to a multi-grade. Multi-grade oils tend to leak better, but on a good tight engine they usually reduce oil consumption. I have heard from several pilots who bought a case of mineral oil for break-in and now have no use for it. One option is to add a quart to each AD oil change.
As to which oil to use, that is a long-standing debate. Multi-grade oils have improved flow characteristics in cold weather, and you can use them year around. But a significant number of engine rebuilders have a slight preference for single grade oils in the warmer months.
If you live in a colder climate or fly into a colder climate, I usually recommend using a single grade in the summer and a multi-grade in the winter. If you live in a warmer climate and do not start your engine below about 40°F, you can use single grade oils year around.
If you operate a radial engine, the smaller ones with displacement less than 1500 cubic inches, they seem to work well with multi-grade oils. However, the large ones, especially if you do any reverse loading by the prop, seem to be better served by single grade oils.
Finally, should you use an anti-wear oil? This depends on how you fly and where you live. If you live in a more humid climate and do not fly regularly, then I would recommend using an anti-wear oil. Since radials have roller lifters, an anti-wear oil will not show a significant improvement in these applications.