Dear Ian;
Check your valve spring pressure too.
This was a common scene on the 'early' engines using double springs in the 50s. It was on all of them though. Sometimes we just installed a new set and sometimes we did what was sugested.
See what's in the pan, look at the cam & then decide.
Also; check the past postings. I seem to recall some counterfiet lifters "in the mix" a while back.
It's a combination of things, not just the oil, I think. Tom may be right on the "by-pass" issue too.
Good luck.
