Well of course evidently his is allready taking it to a couple of good technicians for looking at.
but on the same hand, if you have a broken valve spring or a broken push rod, you are more than likely gonna have a miss or at least have a very rough idle.
with a broken pushrod it will not open the valve in 90% of the cases and cause a missfire and rough idle.
with a bent pushrod the valve will not open accordingly and cause a rough enough idle to think you have a bad plug or missfire
If your spring is broken then the valve in most cases will be open , hit the piston, and you will have a loud knocking noise, hence problem, eventuall piston pop.
Also in most cases with a broken spring the keeper will work its way loose if driven long enough and drop the valve down on top of the piston.
Basically the only thing he probably has is his rocker arms need adjusting at the worst, other than that 0w-40 is totally stupid to run in a 500 cid engine, causes major vavle train noise.
goin to a 15w40 is def not going to hurt our engines and I dont believe he stated what weight he ran , how long between oil changes, what brand he used, how hard he drove it, or if it even idled differently.
so yes changing to a thicker oil in most cases in a hot climate like ga has had the past three weeks will not hurt the motor hence my evaluation
WTF?