Originally Posted by ka9mnx
No it will not be a problem for old flat tappet enines. Not unless the original flat tappet engine has been modified with a high lift cam and higher spring rates.
That is not true, it can still cause excessive wear even on standard well worn in cams, i know a guy who has a Triumph TR4 who was using Valvoline Vr1 20w50 in it for a long time, thought it wa sa bit expensive so swicthed to Castrol GTX 20w50, about a year later the engine all of the sudden starts to have a dead miss and is misfiring and running very rough.
He takes the engine out to look at it and he takes the cam and lifters out, sure enough 3 or 4 lobes were flat...
He had run the engine for over 10 years on Vr1 prior to that.