What caused this

Actually there are quite a few oils that have enough ZDDP in them; you just have to do your home work to see which ones. In the olden days, the ZDDP levels were in the 1300-1500 PPM range. That got reduced to 1200 PPM and cams were not being lost. Then the ZDDP levels were reduced to 900 PPM's for one of the recent service grades, and that is when the problems started.

Those 900 PPM reduced levels are for generally found in the the lighter grade oils, 30W straight and multi-grade, and lower. If the service code is SN, then you have a newer, low ZDDP level oil.

That's why I said "regular motor oil". I think for most of us, that means the readily available stuff. You know, walk into WalMart and grab the Castrol GTX 20W-50 that's on sale (for example). Even that oil, which most would consider "high-performance" is API SN. Yes there are special oils containing more ZDDP. I think I said that too.
:wtf: