What should my oil consumption be?

One of the most often asked questions by pilots is “what should my oil consumption be?”

This is one of those questions that doesn’t have a perfect answer.

I have been to several flight schools with a number of identical aircraft, but when you look at the oil consumption records, there can be a wide variety of oil consumption values between the various aircraft.

So why are there variations? And is lower consumption always better?

There are basically three ways that oil can leave your engine: 

1. The first is through leaks. These are usually caused by bad gaskets or something loose or cracked. Some are easy to fix, but most are difficult to pinpoint and repair.
2. The second is past the intake valve guides. These can sometimes be repaired by replacing the intake valve seals, but if the problem is excessive valve to guide clearance, we are talking major repairs.
3. The final problem is oil going past the piston rings and directly into the combustion chamber.

While the first two sources of oil consumption should be zero or at least almost zero, the piston rings are more problematic.

Most pilots also drive and maintain automobiles and are aware that most late model cars do not usually burn or consume any oil between oil changes. They then reason that an aircraft that burns a quart, say, every four hours has a problem.

A few years ago, Continental Motors changed the ring design on its large 520 and 550 engines. The result was these engines would go up to — or over — 10 hours per quart of oil consumed. The pilots were all very happy.

However, within a few years the engines experienced excessive cylinder wear and then loss of oil control. It seems that air-cooled aircraft engines need a certain amount of oil on the cylinder walls to properly seal the compression rings. 

Liquid-cooled automotive engines operating at lower loads and with the lower cylinder wall temperature do not need this oil consumption to get good ring seating.

There is also the problem of the right oil level.

When an aircraft engine is certified, the manufacturer must supply a great deal of data to the FAA, including the minimum oil level that the engine can operate on safely with no damage to the engine. The manufacturer then needs to specify the maximum oil consumption allowed and the maximum flight duration of each specific aircraft.

So, if you have an engine with a minimum oil level of, say, four quarts, the maximum allowable oil consumption is a quart an hour and enough fuel to go four hours. Then manufacturers are required to add four quarts to the original four quarts, so the aircraft engine must have an eight-quart sump to be certified.

Many times, the increased sump requirement results in excess oil in the sump and the aircraft engine tends to throw out the excess.

That’s why a lot of pilots find that their oil consumption is much higher for that first quart than for subsequent quarts if the sump is operated a quart or so low.

However, this is not true for all engines. For example, the Lycoming O-320H should be operated full. Always check with the manufacturer or rebuilder for their recommendations. 

And then there is the type of oil you use. If your engine has leaks or poor valve seals, your oil consumption will probably go up if you switch from a single grade oil to a multigrade oil. Conversely, if your oil consumption is primarily past the rings, the oil consumption will probably go down if you switch from a single grade to a multigrade oil.

The good news is that after your engine is broken in, it will probably settle down, hopefully, to a reasonable oil consumption level.

The important thing to remember is your airplane engine is very different than an automobile engine, so you have to check your oil level before every flight.