(quote/Ragman): "It will be interesting to see your results with Valvoline GTX. Word has it that they cut back the ZDDP additives because of damage to cat. converters."
^^^this! Feds required motor oil manufacturers to cut back on ZDDP from oils we now get.
Recently I poured 2oz of ZDDP additive at my last oil change & it seems to quiet down the valve train. ZDDP in oil is good for flat tappet motors like we had during "the good old days"...think street rods.
Articles I read says it lubricates the cam lobes as they bang around the tappets. Seems to be working for me but at 2,000 kms, it's too early to tell. I like the way my motor sounds now, though.