The first motor, 1962 pump gas 426 M.W., I dyno tuned lost 30 HP going from Valvoline 30 WT detergent oil to Mobil 1 10x30 WT
It was slightly seeping oil through the intake port gaskets on #2,#5 and #7, those spark plugs where slightly shiny before the oil change. After the first pull with the Mobil 1 they where all (8) oiling bad Synthetics lubes will leak past gaskets that regular lubes won't
I've heard that most motors that are sealed up good will gain some HP and torque changing to a good brand of synthetics lubricants I haven't done any other test on them since back then in 1991, once bit, twice shy