There's a lot of info on the zddplus site that crenwelge linked to. I know they're trying to sell you something but they document a lot of what they claim. They've tested engines with an overdose of ZDDP and the result is, no problems. I'm going to try a bottle with my next oil change along with my usual Castrol GTX 20-50. I have an account with Eastwood and they carry it so I've ordered a bottle and it's on it's way. $10 sounds like a cheap insurance policy to me. Oh, and they mention diesel oils and why they are unsuited for gas engines.

I wish one of the big oil co.s would formulate a blend to meet the needs of the tens of thousands of '89 and older engines still out there and still running but I suppose the sales numbers wouldn't support that.