In a recent column, “For oils, it’s all about the chemistry,” I wrote about the difference between automotive and aircraft engine oils and why you should not use automotive oil in a certified aircraft engine.
I’ve since received two similar questions about adding anti-wear additive zinc dithio phosphorus (ZDP) supplement in experimental aircraft engines.
ZDP supplement containing additives are sold in automotive stores. By using just the supplemental additive and not the complete automotive oil, pilots would not have the concern about ash-containing detergents leading to pre-ignition.The first question was from an old friend, and concerned using the additive in an aviation conversion of a VW bug engine. Here I would not expect any problems with the ZDP additive being used, especially during break-in.
These engines were designed for automotive oils, so should not have any parts containing silver or copper that would be affected by the ZDP additive.
The other solution discussed was the use of the additive STP in the oil. This should also help, because STP contains a low level of ZDP as part of its formula.
The second question came from a pilot with a Lycoming O-360 in an experimental aircraft. Here the answer is more of a gray area.
Technically, the ZDP additive should work IF the owner is absolutely sure that the engine does not have copper alloy exhaust valve guides.
If the engine has copper alloy exhaust valve guides, the ZDP will tarnish the guides and probably lead to valve sticking.
On the legal front, this is another question. For a certified Lycoming engine, it needs to be operated on a Mil-L-6082 or 22851 spec. (SAE 1899 or 1966) oil. These specs call for zero ash level oil, so the use of a supplemental ZDP additive would not be allowed.
However, it is a bit of a gray area because Lycoming does allow the use of STP, which contains ZDP, as an assembly fluid to coat different parts.
If you got into a legal issue with the engine and zinc was found in the oil, you could just claim that it was from the assembly process.
The main experience that I have with aviation oils containing zinc comes from the Phillip X/C II oil that was introduced way back when. It contained a low level of ZDP and seemed to work well, except for the exhaust valve sticking issue with exhaust valve guides that contained copper and the eating away at silver master rod bearings in radial engines. Other than that, I do not know of any major problems, but it was not on the market for too long.
One of the major problems with ZDP is its chemical nature. It works by chemically attacking iron parts and coating them. This coating then protects the part during high shear load operation, like the cam and lifter interface.
The greatest reduction in wear is when an oil is treated at about 1,400 parts per million (ppm) of zinc. So if you test the wear allowed by an oil, the wear continues to decline as you add ZDP to the oil up to the 1,400 ppm level.
But because of its aggressive nature, if you go significantly above that level, the wear actually increases due to zinc pitting of the metal. So the idea that if a little additive is good, then more must be better, does not work here.
Never over treat an additive.