I have received a number of questions regarding the level of zinc dialkyl dithiophosphate (ZDDP) in modern automotive engine oils. It appears that people are experiencing a significant number of camshaft and lifter failures on newly overhauled engines with flat tappets. The failures appear to be caused by a lower level of ZDDP in the latest spec oils.
This does not concern normal certified aircraft engines, which are designed to run on non-ZDDP oils, but may concern some Light-Sport Aircraft (LSA) engines, automotive conversions, or your ground transportation engines.
About 40 years ago, the oil industry began using a standardized rating system for oils. In spark ignition gasoline automotive engines, the classification ratings were SA, SB, SC, and SD. Through the years, the specs have been updated to today’s SN rating, which you will find on almost every quart of oil you buy.
This change in spec never caused a problem with older engines because the newer spec oils usually meet all of the requirements of the previous specs.
But that has changed. The Environmental Protection Agency (EPA) has issued a requirement that all automotive manufacturers must not only meet the emission specs for cars when they are new, but they must also maintain a certain level of emissions over most of the expected life of the vehicles.
Testing showed that the phosphorus from the ZDDP in the motor oil was poisoning the catalysts in the exhaust converters, which resulted in failing the emissions tests at high mileage.
To help solve this problem, the manufacturers issued the ILSAC GF-1 through GF-5 specifications a few years ago that limited the allowable amount of ZDDP in the automotive oils sold for their new vehicles. The limit went from about 1,000 parts per million (PPM) for GF-2 oils down to around 600-800 PPM for GF-5 oils.
The spec is even more confusing because the limit is in percent by weight of phosphorus and different ZDDP additives have varied levels of zinc.
But the bottom line is if you use a SM or SN oil with the ILSAC GF-5 label, it will have the lower limit of around 600-800 PPM zinc.
In a flat tappet lifter engine, the point where the camshaft contacts the lifter is the highest load point for the lubricant. ZDDP additives work by attacking and coating the cam and lifter face with a microscopic layer. Then when the lobe starts to open the valve, that layer is sheared off. This is called sacrificial lubrication and it greatly increases the load carrying capability of an oil.
Tests have shown that the amount of wear on a cam decreases as the amount of ZDDP is increased, up to around 1,400 parts per million (PPM) zinc. Above that level, the wear will actually increase due to zinc pitting. So all automotive oils have contained around 1,400 PPM of zinc additives for many years.
The new SN spec works well in new engines because they all have roller valve lifters that do not need the higher ZDDP level. And most older cars are well broken in and can usually live with the lower level of ZDDP.
However, if someone overhauls their engine or replaces the camshaft, the lower level of ZDDP will not provide enough anti-wear characteristic to protect the new cam and lifters.
So what do you do? One solution is to replace the cam with a roller cam and lifters. If that is not an option, you will need to check with your lubricant supplier to see if it has an oil in its line that still has 1,200-1,400 PPM zinc.
Putting an additive with ZDDP may not work, because it could put the zinc level over the limit, which could increase the cam wear. So it’s important to stick with the right oil.
Now this is were the confusion really kicks in. I called or went on the website of four different oil companies, and got five different answers. They all agree that any oil with SN and GF-5 label will have only 600-800 PPM zinc.
But some oils have a SM label and not the GF-5. What are the zinc levels of these oils? Most thought they would have 1,200-1,400 PPM zinc.
Also there was some confusion about heavy duty engine oils. Most agree that unless the oil has an ILSAC GF-3 to -5 label, that it probably would contain 1,200-1,400 PPM zinc. I have been using a heavy duty 15W-40 oil and have not had any problems.
Another option would be to buy a specialty racing oil or an oil recommended by your mechanic or camshaft supplier.
I would like to restate that you should never use an oil containing ZDDP in a certified aircraft engine as it can cause valve sticking, bearing failure, and possible pre-ignition. This information is meant for the VW, Corvair, and other auto engine planes and ground vehicles that may have a problem with low zinc oil.
Now I do not have a problem with the EPA requiring long-term testing. I think it is a good idea. I do not have a problem with the auto manufacturers changing their spec. But why they do not bother to tell the automotive service community that there may be a problem does bother me. I guess they expect us to just buy new cars and not fix the old ones.