Mixing Piston Aviation Oils: Will topping off with a different brand, weight, or type harm the engine?

Mixing Piston Aviation Oils

Will topping off with a different brand, weight, or type harm the engine?

Photo by Bill Coleman, Phillips 66 2003Lubricating oil can cause a lot of friction among piston engine aircraft owners. Oil myths and legends abound. The slippery stuff has been blamed for everything from a ruined engine to post-nasal drip, while other pilots brag that their oil gets their airplane 500 hours past TBO.

Most pilots prefer a specific brand, but they can't always defend their oil choice with logic and reason. That's why it's so difficult to consider another brand - even in a situation like this:

A bad situation
It's getting dark and the pilot is still three hours from home on a long cross-country flight. On a fuel stop, he discovers his engine is 2 quarts low. His favorite brand of oil is not available, they only have brand XX. What to do?

We asked the pros - FBO operators, engine overhaulers, oil experts, and others who care a lot about protecting their customers' aircraft.

All advocate taking the safety-first approach and add the needed oil, even if it isn't the pilot's brand.

All aviation oils compatible
For Walt Silveira, Phillips 66 aviation oil product technical manager, the answer is simple - "Stick with a brand the pilot prefers when you can, but any approved aviation piston engine oil brand for emergency top-offs. Topping off the oil level is much better than flying low on oil, even if it is not the preferred brand or the right viscosity."

Silveira says the proof is printed on the container's label. All aviation OEMs (original equipment manufacturers) - including Lycoming, Continental, and Pratt & Whitney - use the same SAE (Society of Automotive Engineers) tests and specifications to assure that approved oils meet the stringent standards to protect their aviation engines.

"All aviation piston engine oils are compatible," Silveira says. "Whether mineral-based, synthetic blend, straight grade, or multigrade, all aviation oils are compatible and can be mixed without harm to the engine. I'm not recommending that owner/operators make their own oil by mixing a quart of one type of oil with a quart of another type oil during oil changes, but rather when a situation arises where you are down on oil, the decision to add a different brand or grade is a good one since this will not cause any engine problems," Silveira says.

How about switching from an all mineral to an AD oil?
According to Richard Fowler, America Aircraft Engines in Tulsa, Oklahoma, "Can I mix an Ashless Dispersant (AD) oil with a non-AD oil? The answer is yes and both types of oil are very compatible since similar base stocks are used in both types of oils. AD oils are not designed to clean up deposit and sludge buildup that is already in the engine.

"Also, customers ask me if they will see any performance difference on my engine if I totally switch over from a non-AD mineral oil to an AD mineral or semi-synthetic oil."

There will be no negative effects to switching from a straight mineral to an AD oil, regardless of the number of engine operating hours accumulated.

"AD oils will not remove past accumulations of lacquer and varnish or hardened sludge. All oils will not cause sludge to move, blocking the oil galleys," Fowler says.

"When switching from mineral to AD oils, a quicker than normal darkening of the oil may occur on the first oil change. This poses no danger to the engine and means the oil is suspending a small amount of deposits that have not solidified."

A little planning, common sense
"Of course it's a good idea for a pilot to plan ahead and always carry a spare bottle or two of his favorite brand. That way, he won't have to worry about mixing oil," Fowler says.

"I try not to mix oils," admits Jim Peterson, retired Navy and corporate pilot and technical editor for Cessna Owners Organization and Piper Owner Society. "That's for no scientific reason whatsoever, but I always carry a supply of my own oil - just in case."

If you do have to decide about adding oil, rely on common sense. Use oil as much like the pilot's preferred brand as you can find - a similar viscosity range multigrade, similar single-grade weight, and similar mineral or synthetic composition.

"Pilots should be confident that all approved aviation oils are compatible," Silveira says. "They all meet SAE aviation specifications, and all have been approved by aviation engine manufacturers. As long as they are aviation-approved, they have demonstrated compatibility."

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