What oil will GA use with unleaded fuels?

When general aviation completes the transition to unleaded fuel, what oils will we be using in our airplanes?

First a little background. In the early 1970s Shell saw a need for a multi-grade oil in the GA market.

It did extensive research on the matter and developed a full synthetic version of the old stand-by Aeroshell Oil W100. After engine tests, the oil was sent out to several locations for flight evaluations in real world conditions.

The first feedback was very positive.

For example, the oil completely eliminated any oil cooler congealing. But then at around 600 to 1,000 hours, oil consumption started to go up. I am not talking about going from 8 hours a quart to 7 hours a quart. I am talking about going from 8 hours a quart to 1 to 2 hours a quart.

Shell representatives immediately went out to the test sites and started pulling cylinders. All of the cylinders pulled looked the same with a gray epoxy-like substance that completely coated the piston and ring belt area.

The analysis of the substance showed that it was basically lead by-products of combustion.

It was apparent that pure synthetic base stocks are excellent lubricants, but very poor solvents that do not absorb the lead by-products of combustion.

This has been generally accepted as fact, however a couple of oil companies subsequently marketed full synthetic oils to general aviation to reinforce that fact and found the same results.

So, if we go to only unleaded fuels, will we all be using full synthetic aviation piston engine oils?

I think the best answer would be “definitely possibly maybe or at least could be.”

There are a number of factors that would need to be considered.

The main consideration is whether full synthetic oils offer any definite advantages over a semi-synthetic oil or even a mineral-based oil. Included in that consideration must be the fact that a full synthetic oil is going to cost more than a mineral or semi-synthetic oil.

Full synthetic oil could provide better cold temperature starting, better high temperature protection, longer oil changes, and better wear protection.

Better cold temperature starting may not be a huge thing because I doubt that the engine manufacturers will lower the temperature at which pre-heating is needed.

Part of the reason for this is pre-heating is not just to get the oil to flow. It is necessary to increase the bearing clearance for the cam and crank because the aluminum crankcase shrinks faster than the iron crank and cam as the temperature drops.

The same thing for high temperature protection: Higher oil temperatures would mean higher overall engine temperatures.

Whether there will be longer times between oil changes is a bit of a gray area. It really depends on how much the aircraft is used.

Most private aircraft are flown about 100 hours a year, with oil changes advised at least every 50 hours or four months, whichever comes first.

The University of Illinois flight school has been using 100 hour oil changes for years with 100LL. Airplanes that are flown a lot can use those longer oil change periods, but low usage aircraft — like those flown just 100 hours a year — will have rust problems while sitting for long periods of time.

Another concern is that a full synthetic oil will leak more than other oils.

And, finally, pure synthetic oil that did not absorb lead by-products of combustion may not absorb the dirt and acids produced during normal operation even with an unleaded fuel.

My guess is that we will stay with the products that are currently available.

And then, eventually, some oil company executive will try to capture more market share and will introduce a full synthetic aviation piston engine oil. And the marketplace will decide whether it is a good idea or not.