I'm running 5w-30 full synthetic Valvoline in my 283, still has the rope rear main seal with no leaks, no smoke, no lifter noise, 35psi hot wound out. 1957 shop manual calls for 10w-30 for the major temperature range.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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