Can I mix different brands of oil in my aircraft’s engine?

I recently received an email from a Brian Favero, who owns a Cessna Turbo 310Q with a Continental TSIO-520 engine. Recently his mechanic changed oil brands, switching from Aeroshell Oil W100 Plus to Phillips 66 X/C Aviation Oil 20W50.

Brian has a case of the Aeroshell oil and was wondering if he can mix the Aeroshell oil with the Phillips oil.

There is conventional wisdom that you shouldn’t mix different brands or even grades of oil in a mid-time engine.

Some of this comes from the original introduction of Aeroshell Oil W in 1958. At that time straight mineral oil was the only product that was approved for aircraft engines. The new Aeroshell oil was the first ashless dispersant aircraft engine oil.

When the W oils came on the market, a lot of high-time engines that had a considerable amount of carbon in the engines were switched over to the new oil. In some cases, the ashless dispersant in the W oils would loosen up enough carbon to plug the screens or other passages and cause engine damage.

Once the W oils became more widely used, the problem went away. But the issue was well-known in the aviation community and became part of “aviation conventional wisdom.”

This was exacerbated by problems in the automotive world where some different chemistries were tried by some oil companies that did not mix well with other products on the market.

In addition, there is the fact that single grade and multigrade oils can have a different oil consumption rate in an engine. For instance, multigrade oils leak more, but also work better in the ring belt area.

With all that said, let’s go back to the original question: Are the two oils compatible?

Yes, they are.

All oils that are qualified against the Mil-L-22851 specification or SAE J-1899 standard must pass a compatibility and miscibility test with all other approved products before being approved. Miscibility refers to the ability of one liquid to completely dissolve in another liquid.

In fact, almost all approved products that meet this spec have very similar formulations, so aircraft owners can switch from one brand to another with no problems.

Where you can see some issues are in the different viscosity grades.

For example, if you add a single grade oil to 20W50, you will lose some of the low temperature properties of the multigrade oil.

The big advantage of a multigrade oil is that at low temperatures, it will result in faster cranking and a better chance of starting. And — more importantly — multigrade oils will flow to critical bearing surfaces quicker, which usually results in less wear.

If you live in a colder climate or fly into a colder climate, the addition of the single weight oil will reduce those advantages of the multigrade oil.

What I would recommend is that you save the single grade oil until next spring and then use it on your next oil change.

Also be aware that your oil consumption may vary with the different viscosity oils depending on how your engine is using it.

Is an Oil Additive Needed?

Brian had another question: Does he need to add an oil additive to the Phillips oil?

The Lycoming LW-16702 additive in the Aeroshell Oil W100 Plus is designed to protect the camshafts in Lycoming engines where the camshaft is located above the crankshaft.

In this location it is in the colder part of the engine, which can lead to condensation forming on the cam during prolonged storage.

In Continental engines, the camshaft is located under the crankshaft, so there is less condensation.

In addition, once the engine starts to turn, oil is dripped down on the cam to lifter interface, reducing “dry start” wear on the lifters.

The bottom line? Brian’s engine should be fine without the additional additive.