I've ran and used Valvoline 5W20Wtnon synthetic for a lot of years (15+yrs)in my street and strip as well as race only BB Mopar bracket motors making well over 600 HP with no problems so far. I do run loose clearnaces all thruoghout the motor, .003+ on the rods, .0035+ on the mains, .001+ on the wrist pins and at least .001 more than recommended on the piston to cylinder walls clearnces at room temps, .60 to 80 F I shoot for(trim the oil pump bypass spring to set the WOT pressures) the 10 lbs per 1000 RPM rule with hot oil,160+F on the engine dyno with the Valvoline 5W20Wt oil