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Back in 2006, a new motor oil warranty category for gasoline engines was released. Designated GF-4, this new warranty specification was licensed by the American Petroleum Institute (API) and superseded all previous specifications. Historically, when a new specification such as this was released, all older cars built under a previous specification would be able to operate on the new one as well without any loss of lubricant protection, as the newer specifications normally provided at least the same level of protection as previous ones, and generally have provided improved protection as research developed newer and better detergents, additives, etc. However, that didn't happen in this instance, and it wasn't long before problems started being reported with engines in older cars.
For many decades all of the major oil companies used ZDDP (Zinc Dithiophosphate) in their motor oil formulations. ZDDP is an excellent anti-wear agent, and for years was inexpensive for the oil companies to purchase.
With the introduction of catalytic converters everything began to change. When first introduced, catalytic converters contained things like platinum and palladium, which were expensive to buy, and have continued to become more expensive as the years have gone by. Due to this, the original equipment manufacturers (OEMs) changed the composition of catalytic converters to make them less costly. And with that change came the need to reduce the levels of sulfur, phosphorus, and zinc in motor oils because they will eventually destroy catalytic converters on new cars built with the new materials. When the EPA mandated that ZDDP be removed from all conventional and synthetic oils produced domestically that sought to meet the GF-4 specification, apparently no thought was given to what might happen to older engines that require ZDDP.
ZDDP protects older engines by creating a film on cams and flat lifter contact points. This film, consisting of zinc and phosphorus compounds, provides a sacrificial wear surface that protects the base metal of the cam and lifter from wear.
Additionally, ZDDP alters the surface characteristics of bearings and journals to prevent metal to metal contact, and reduces the tendency of critical engine components to scuff or gall under heavy loads.
There are a few motor oils on the market today that still have higher levels of zinc in them.