But there are those who do have aggressive aftermarket cams and high pressure springs, or people just want extra assurance, like me. I use a heavy duty diesel engine oil with typical zinc level of >1200 ppm. Sholoff posted some brands a couple of posts up, any of those are good oils. The new CK-4 specification for HDEO's raises the performance level up even higher, but beware of oils that list both API CK-4 & SN service classification, as they now are required to meet the lower zinc levels of the SM category. Stick with an oil that is CK-4 only and you'll be just fine.

I'm NOT a fan of aftermarket oil additives, like ZDDP in a can, as they can (and probably will) upset the balance of the chemistry of the additive package, which includes other surface acting chemicals besides ZDDP, and just mixing the unknown chemicals together with no way to know how they will react is looney, no real chemist worth his diploma would approve.

Also, just to clarify about ZDDP, it is, as I posted above, a surface acting chemical. It has a polar head (seeks to attach itself to metal surfaces) and an oil soluble tail. It reacts chemically with the metal surface due to the heat of operation to form a glass like layer (what we geeks call a "tribo film") on the rubbing surfaces of the engine, to provide a self sacrificing layer. When the parts rub together this layer will rub off, preventing metal to metal contact. As the layer is rubbed off, additional ZDDP in the oil will plate out on the surfaces to replace it. The rate at which the film rubs off is related to the severity of the load (aggressivenes of cam and pressure of the valve springs). A mild or medium severity cam and low to medium pressure springs will remove the film at a lower rate than an aggressive cam with high spring pressures.

Now this is important. The tribo film will only get so thick, no matter how much ZDDP is in the oil. Putting more ZDDP in the oil does will not provide a thicker film. Let that sink in for a moment. More ZDDP does not equal a thicker tribo film. What it means is the oil can protect the surfaces longer than an oil with lower levels of ZDDP. In theory, if you only put a few hundred miles between oil changes, even when using a modern SN gas engine oil with low levels of ZDDP, you will probably never deplete the ZDDP out of the oil and never have a wear problem. Of course, that depends on the aggressiveness of your cam and the pressure of the valve springs, and how hard you drive it.