Piston engine oils - can one use car oil?

For what it’s worth….
When I were a bit younger a friend bought a used car. 16 valve GTE Astra. 30,000 miles.
He changed the Oil to Mobil 1.
That car took so much of a thrashing that he destroyed two gearboxes and needed new tyres every 6 months. Absolutely no exaggeration.
After 3 Years and a clock of 150,000 miles.
That engine still purred and ran beautifully.
All of my cars (Inc a couple of high performance) have been run on Mobil 1 since, with very happy engines.
Not brave enough to swap out the AeroShell on my a/c though.

Mobil 1 has other unofficial uses… See this high oil consumption thread

Peter wrote:

Using the Mobil 1 0W20 method

Just saw this:
https://www.mobil1.co.uk/synthetic-engine-oils/esp-x2-0w-20.aspx?tabs=specifications#tabs warning…
Apparently, your method is more widespread, so that Mobil saw a need for a warning ;-)

“Mobil 1 ESP x2 0W-20 is not recommended for 2-Cycle or aviation engines, unless specifically approved by the manufacturer.”

This is the correct link

Warning :-) [ local copy ]

Maoraigh wrote:

Thread drift: local grapevine says local suspicious guy had guarentee repair on his very low mileage, new, BMW refused as it was run on heating oil.

You mean red diesel, right ?

so that Mobil saw a need for a warning ;-)

Sure; one cannot just put Mobil 1 into a Lyco-type aero engine. You will eventually get metallic deposits which can produce pre-ignition. One has to do it in a particular way, for a very limited period, and obviously at one’s own risk, etc.

There is a guy named Mike Busch who can tell everything about aviation oils.
In essence:

• aviation engines aren’t comparable to modern automobile engines.
• aviation engines are produced using bigger tolerances. The piston doesn’t fit as tightly into the cylinder as it does in a modern automobile engine. There are at least 40 years of technical innovation between our old aviation engines (which essentially are old Caterpillar tractor engines) and what we see in a modern car.
• therefore, aviation engines see more wear and abrasion, resulting in micro metallic residue which needs to be bound and kept by the oil
• automobile oils are made to lubricate, but they aren’t able to bind metallic residues, which will end up in the engine and wear it down.
• automobile oils are designed to stay in the engine for a long time (intervals of 30.000 km or 2 years). They don’t need to bind any residues, because there aren’t so many, as the engines are produced in a much more precise way.
• aviation engines need mineral oils, which have a different molecular structure and are much better suited to bind the metallic residues
• the downside of mineral oils is their shorter lifetime, which means they should be changed after 30-50 hours.
• even mixed oils – half synthetic / half mineral – aren’t really good, as their mineral part wears down, leaving the synthetic but contaminated part which is even worse.
• also, since aviation engines still work with leaded fuel, the mineral oils are able to bind the lead residues from the fuel burn process.

So, in essence, automobile oils are pretty bad for the classic aviation engines. I am not talking about the Rotax engines, or Thielert Diesels, because they are much more modern or are converted automobile engines.

So, the best oils for Lycoming and Continental engines are single grade mineral oils, with no additives. The Mobil Oil you are talking about here is good for Lycoming, but bad for Continental, because it contains an additive designed to sprinkle the overhead camshaft in the Lycoming, but at the same time destroys the Starter coupling of the Continental.

So, it’s a bit of a science and I really, really, reeeaaaallllyyyyy suggest to familiarize yourself with the savvyaviation material on Mike Busch’s website: https://www.savvyaviation.com/

Result was, we have completely changed our oil and the change cycles in our Beech Bonanza, and we are using full mineral single grade oil, either Philipps Aviation XC66, or similar, 80 or 100W, and we change it after 25-35 hours.

While I agree with most of the above, I would summarise it slightly differently. Aircraft engines are air cooled and run mostly on leaded fuels. That drives most of the above.

One of my pet peeves is using the word tolerance to mean clearance. They are two different things

In the context of this discussion:

Clearance: the nominal gap between two parts specified in the detail design drawings.

Tolerance: the acceptable range of clearance between parts in an assembly in relation to the nominal clearance, a manufacturing & quality issue. In other words the variation from the nominal design clearance that is considered tolerable.

Most air cooled engines use greater piston to cylinder clearances than water cooled engines, particularly when using a large diameter aluminum piston in a steel cylinder. That has nothing to do with the manufacturing tolerances and is not indicative of bad design. It just means the parts are designed to fit when the engine is warm and the piston has expanded relative to the cylinder, reducing the clearance.

Aircraft engines have more in common with large displacement air cooled motorcycle engines than “Caterpillar tractor engines” which are water cooled diesels. For example, BMW has had lots of trouble sealing up cylinders on air cooled twins with large bore 600 cc cylinders, they often burn oil even with wear coated aluminum bore, for the same reasons as an air cooled aircraft engine. In the case of the aircraft engine, with large air cooled cylinders you in exchange get the benefits of lower weight, reduced complexity, and increased fault tolerance.

Oil contamination by leaded fuel is clearly an additional design requirement for aircraft engines, assuming they’re running on leaded fuel – which is not a necessity for most of them.

I stand corrected, what I meant was clearance. I hope the context and meaning was clear nevertheless, since I can’t edit the post anymore. Of course it all comes down to air cooling as a root cause, but that doesn’t explain by itself why aviation engines, oops sorry, air cooled aviation engines need mineral oils.