These questions are always best left to the engine builder first.... We don't know what the bearing clearances are, nor the oil pump/pan/pickup setup. Personally, for a street car, I would stick with a synthetic after break-in, but I would'nt use 20w50 on the street especially in cool weather unless the machinist said I have too. A lot of people don't realize, any potential oil consumption is increased by use of heavier weight oils. HTH, Lefty71
The builder of my 512 is a top quality builder. He surprised the heck out of me when he said to use 20-50W Penn Grade. His recommendation became a little more clear when I read his build sheets and the bearing and rod clearances. Later I learned that the Penn Grade is the same as the old Kendall racing oil. That was the best oil I ran in my bracket motors back in the day.
So you are just installing your 512 in the Car ? No Dyno'ing ?
What are the internal Bearing Clearances ?
Where is the spec/build sheet on the Engine ?
Did your 'builder' Machine the Engine as well or just 'assemble' the Engine ?
What if the rear Main Seal leaks ?
Water Leaks ?
My apologies here but just say'in.... if it's an 'all roller' Engine and you don't even know the Bearing Clearances from a "Build Sheet".... let be the V/Spring Seat Pressure or Rate that was used ? Cam End Thrust ? Ring End Gaps ? Pin-Fit ? Valve Lash ? .... and on and on and on... What Oil to use could be the least of your worries ?
you still need Zinc in the Oil for patent break in on the rings.... because I'll be damned if you know the Hone/Plateau procedure used either ?
roller affords you the ability to pretty much run any modern oil after break-in, synthetic or conventional. Try to stay above the 10psi per 1000rpm (hot) rule and adjust your viscosity accordingly. I would guess 20w-50 would not be ideal unless the bearing clearances are large.
I have a solid flat tappet and only use conventional 10w-30, doesnt dip below 15-20psi at 1000rpm at temperature.