I, too, think shorting oil is a dumb idea. But knowing where the engine is happiest with respect to the oil level can be helpful because you can track oil consumption rather than that oil expulsion.

I’ve always done oil analysis on my airplanes, and the oil consumption trend is important to watch, too. They analysis companies always ask you how many quarts you added since the last change. Plus, if you’re constantly added fresh oil because it’s getting expelled by the engine, it can skew your spectrographic analysis figures.

But you’re right, job #1 is ensuring the engine has sufficient oil to operate properly.

The quantity of oil only plays a bit part in the cooling of the engine. The surface area of the oil cooler(s) and the airflow across them is all that matters as far as how much heat can be removed from the engine by the oil. The importance of the quantity is that a larger quantity can carry more heat internally during transitions such as a suddenly shut throttle at high speed or a suddenly opened throttle on a hot runway.

One thought comes to my mind: It is probably quite hard to design a wet sump oil system that will work well both full and half full. The CFR says it must work half full, so perhaps the rated capacity on many of these powerplants is knowingly a bit beyond what the oil system will happily and efficiently contain, so that when “half” full the system works as advertised.

a side note: it would have been nice if the FAA had changed part c) to reflect the existence of safe and effective PCV systems that don’t spew oil overboard.

My experience with Lycoming O-340, O-360, and IO-360 engines is that while most have an 8 quart maximum capacity, they tend to expel anything much over 6.5 or 7 quarts. When one of these engines has an oil change, it makes little sense to fill it so that it reads above 7 quarts.

The SR22 that I sometimes fly seems pretty happy in the 6.5 to 7 quart range, but with 310 ponies under the cowl, I’d be nervous operating below 6 quarts. The AFM for the SR20 that I instruct in specifically says its 200HP engine should not be operated with less than 6 quarts of oil.

The O-360 in a TB-10 I instruct in seems happy at no more than 6 quarts, though the max capacity is 8 quarts.

On the other hand, I don’t like the idea of “shorting” an engine on oil or flying 100 hour intervals between oil changes, just to save some money. Oil is relatively cheap, after all. Being stingy with oil makes me nervous, especially in a twin that has unfeathering accumulators. I actually heard an instructor tell a student to add just a half a quart of oil to an engine on a Duchess before they were about to begin a long cross country trip, which seemed foolishly frugal to me. I know some pilots and instructors who will fly with one of these engines reading as little as 5 quarts. Not me!

If I owned my own aircraft, I’d probably use one of the synthetic-blend oils and do engine oil analysis at each oil change. And the thought of me owning an aircraft is pretty remote. Like most professional instructors, I’m so broke I can barely pay attention!

Another excellent post!