As pointed out by the others, the problem is with the
oil. The new spec doesn't have the additives needed for flat tappet cams. You need to either run standard passenger car spec
oil with an additive, or switch to diesel
oil. There is no danger with diesel oil unless you have fuel injection.
Believe it or not, a simple bottle of STP contains enough
zinc to bring modern passenger car oils up to the same level of
zinc as diesel oil.
I've recently switched to Isky Rev-Lube on the cam and Rotella 15-w40 for break-in after twenty years of 10w-30 and moly paste because of this issue. Rotella also makes a 5w-40
synthetic that would be a good choice after 1500 miles of conventional oil use.