Quote Originally Posted by DennyW View Post
A
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4. This from the Brad Penn Oil Company:
There is such a thing as too much ZDDP. ZDDP is surface aggressive, and too much can be a detriment. ZDDP fights for the surface, blocking other additive performance. Acids generated due to excessive ZDDP contact will “tie-up” detergents thus encouraging corrosive wear. ZDDP effectiveness plateaus, more does NOT translate into more protection. Only so much is utilized. We don’t need to saturate our oil with ZDDP.

And there you have it...I still use Havoline for the older engines, and seems to work just like the old days.
I have used Castrol GTX 20-50 in my 429 for as long as I can remember (40+ years) but also use Havoline 20-40 in my other cars.
So far all good.

Having said that our 'newest' car is a 1988 model!