[rotorwrench]

Anything more than 1200 ppm starts to become detrimental to the engine parts. This was where the API set the limits for good reason (for automobile engines). ZDDP has corrosive properties so don't get too carried away with it. Most diesel engines have larger & heavier duty parts than car engines so they did use higher amounts. The latest API CK-4 rating has reduced levels of ZDDP. Some of them are skirting the API CK-4 rating by listing the engine protection test results. They don't pass for that rating yet but if they eventually do then they won't have much more ZDDP than car engine oil has. They will just use other additives like they have for years. Diesel truck oil is overkill for most automobiles. The stuff helps with the high perfomance engines that started to show up in showrooms in 1958 but the earlier designs really would only need it for initial break in. After that, they would work just fine. The old flatheads don't need it at all. Rompum stompum engines with high compression and high lift, flat tappet cams will be the only ones that benefit from high ZDDP levels. Roller cams don't need high levels.