What’s in a word?

In almost every area of interest, there are buzz words that stand out.

For example, in piston aircraft and cars, the word Turbo, like a Turbo 210, creates a great deal of excitement and interest.

In the lubricants field that special word is synthetic. No matter what you say after that, the word synthetic implies that the lubricant is special and will solve just about any problem you might have.

But what does synthetic really mean?

Basically, a synthetic is any product that is produced by synthesis as opposed to by natural origin.

In the lubricant field, there are generally two different synthetic base oils used. The first is synthetic hydrocarbon and the second is a synthetic hindered ester oil. 

Synthetic hydrocarbon oils are normally poly-alpha-oliphins (PAO) and are sourced from naturally occurring crude oil.

In these oils, the hydrocarbon molecules are basically disassembled and then rebuilt back in a specified way to give the oil tightly controlled characteristics. This gives PAOs better and more uniform cold weather properties, a much higher viscosity index, and some improved high temperature characteristics compared to normal hydrocarbon oils.

The synthetic ester products are pure chemicals usually produced as a reaction product of an acid and alcohol stream. They have super high and low temperature properties.

For example, the rear bearing on an after-burner jet can reach about 600°F during takeoff. Ester products cannot be used on a continuous basis at this temperature, but they will lubricate the bearing if the flow rate and the low residence time is maintained. 

So why aren’t ester base oils used in piston aircraft lubricants? Cost is a significant factor, but the big reason is seal and component compatibility. 

Aeroshell Oil W is a semi-synthetic PAO blend. By agreement in the industry, a semi-synthetic has a minimum of 50% synthetic base oil. The remaining part of the base oil package is mineral oil.

In the automotive world there are mainly partial synthetics, which have no particular limits. The oil manufacturer just adds PAO or a similar component to their blend until it meets the viscosity limits they are shooting for.

How will unleaded fuel impact my aircraft engine’s synthetic oil?

With all the talk about an unleaded aviation fuel on the horizon, some pilots are wondering how that new fuel will impact their use of synthetic oil in their aircraft.

The general knowledge in the aviation community is that full synthetic oils cannot be used in piston-powered aircraft because the synthetic base oils will not absorb the lead by-products of combustion. And that is true.

I know that many pilots think that synthetic oils last forever and will solve every lubricant problem and never break down.

And it is true that in a properly tuned aircraft engine, mineral oil almost never breaks down unless the oil temperature gets over 300°F. 

So, what is the difference between the two oils? The big advantage for the synthetic oils is cold temperature flow and maybe a slight advantage in an overheated engine.

So, in an unleaded environment, will synthetics last forever — or at least a lot longer than present day mineral-based oil? 

Possibly for some high usage fleets, but generally not for the average private pilot. 

Why?

The problem is rust. A big reason we change the oil in our engines is to remove contaminates like rust particles, rather than because the oil broke down.

I have seen a lot of comparisons between mineral oil changed regularly and synthetics with extended oil changes. And unless it is in high usage aircraft, the frequent changes always look better, with less wear and better chance of reaching TBO.  

The bottom line is I do not think much will change in lubricant recommendations if lead goes away. Full synthetic oils may be offered for winter operation, but frequent oil changes are still going to be the secret to a long trouble-free engine life.