All depends on bearing tolerances. I had old school clearances in the drag engine and ran 20w50 racing. My street GTO with tight tolerances I run 5w30 Valvoline Conventional (translation = old fashion) oil. This gives me 70 pounds cold start up in cooler weather and 65 pounds with engine at operating temperature at 1,900 cruising RPM. The 10w30 I started out with was 80 pounds cold so I dropped the cold side down a bit. If I found that running temp oil pressures were a little low I would have went with 5w40.
In short, use an accurate gauge and respond with viscosity changes to what it is telling you.
Also don't have anything against synthetics and just haven't made the move.
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Mick Batson
1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress.
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