Valve lifter question.

Car: 1972 Duster 340
Engine: 1992 Commando 360/300 crate
Me: Minor to moderate auto repair and maintenance; but not an engine builder.

Hi folks,

My 360 has a lifter noise. When fully warm I get a tapping from the drivers side - think one lifter is off. Once warm it does it most of the time, idle or cruise. Going from 10W40 to 10(or20)W50 Mobil-1 oil helped out a bit.

I tend to think that it was cause by a radiator that was inadequate for the first couple thousand miles that caused the condition, because the engine was running a bit too hot back then.

The engine has been making this noise most of it's life since new - also has been leaking a very small amount out of the rear main seal since new.

Pretty good engine on the whole, but since I am new to this site, I thought I post my question here in my attempt at getting thoughtful comments / advise.

Thanks in advance and best regards to all,