The No. 1 question of oil experts: How often should oil be changed?

It’s the number one oil-related question of all time: “How often should I change my airplane’s oil?”

Whenever I give a talk, I usually ask the pilots in the audience what they feel is the best oil drain interval for a Lycoming or Continental engine. The answer is always “”50 hours on engines with an oil filter and 25 hours on non-filter equipped engines.”” This is right out of the Lycoming and Continental service bulletins and is almost universally accepted by the aviation community. Unfortunately, many pilots get to the 50 hour part, stop reading and miss the most important part. The engine manufacturers’ entire recommendation is 50 hours or four months, whichever comes first. (In the past, the TCM bulletin stated 50 hours or six months. However, the people I have talked to at TCM recommend changing at four months.)

The 50-hours recommendation is a very good number for aircraft that are flown almost every week and more than 200 hours a year. But we all know that most private aircraft are flown around 100 hours a year, with many of them flown only 50 hours or less. In addition, many of these aircraft sit idle for weeks or even months at a time. For these low usage aircraft, the four-month rule is significantly more important than the 50-hour recommendation.

The reason for the more frequent oil change recommendation on low usage aircraft is quite simple. When an aircraft engine sits idle, it rusts. I have taken used camshafts and lifters, which were first cooked in used oil for several hours at 200?F, and placed them in a controlled environment at 100? with high relative humidity. Within days, you could observe small amounts of rust forming on the lobe surfaces. After a week, the amount of rust was significant on all of the lobe and lifter surfaces.

Obviously this test was more severe than normal service, however whenever your airplane sits for more than a week in a high humidity climate, rust will start to form on the rubbing surfaces. When you start your engine, this very fine rust (iron oxide) material will be washed away in the oil stream. The filter will pick up some of the rust, but most of it is too fine and will remain in the oil. As you continue to fly on this oil, the level of rust continues to increase. As the level goes up, the oil becomes more abrasive. It becomes a lapping compound on your cam and lifter surfaces, which in turn starts the wearing process. Over time, this will lead to a failed camshaft.

By changing oil more frequently, you remove the rust with the oil. This results in a significantly less abrasive oil in the engine, which leads to a better chance of reaching full TBO.

If you live in a very dry climate, the rusting activity will be significantly less than in a high humidity area, so the oil change interval is less important.

Also, Continental engines with the camshaft under the crankshaft are less prone to rusting than the Lycoming engines, which have the camshaft above the crankshaft. When an aircraft with a Lycoming engine sits idle, air comes in through the cowling, which cools down the engine. With the cam in the coolest part of the engine, moisture will condense on the cam surface, which starts the rusting process more quickly.

Another important factor is your oil temperature. Moisture will condense in your engine just from normal heating during the day and cooling at night. If you do not get your oil temperature up to about 180? in flight, you will not boil off the water, which in turn will increase rusting activity.

You are probably thinking that someone who worked for an oil company would recommend more frequent oil changes so that more oil is sold. However, the choice is really quite simple: You can spend more on oil or you can spend more at the engine shop. Oil is a lot cheaper than engine parts.