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Regarding ZDDP, oils did not contain ZDDP when these engines were developed and in service in the 1930’s. all modern gasoline oils have it. The level was just reduced a few years ago to make the oil more compatible with catalytic converters. I am not sure how ZDDP is added into the oils when they are formulated so I am not sure of the efficacy of a bottle poured in by a consumer. The only readily available information is provided by manufacturer of the after market product. Which makes me a bit suspicious. In period literature there is no discussion of excessive premature valve train failures with our engines and this was during a time when ZDDP had not even been invented. To me it has a little bit of “snake oil” aura surrounding it. The proof that it works is a bit like an elephant repellent in Illinois.. “since I started using it I have not had one elephant in my yard.”I’ve only been a licensed mechanic for 48 yrs. I will probably get smarter in the next 48. It is your car and your engine. Mostly just drive them and enjoy them. Regards, Tom
Some good information at this link, especially post #11…https://www.corvetteforum.com/forums/c3-tech-performance/3402561-vette-magazine-all-about-oil-article-march-2014-a.html