Originally Posted By: spasm3
If he does not know the main and rod clearances, could he use the oil pressure gauge as a guide? Try some 10w 30 and watch the pressure as it warms up, stopping to check pressure idle pressure at various stages of heating up. If the pressure drops too low at idle dump and go with the 40wt.
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That's how I would go about gauging what weight oil an engine likes/will tolerate. If idle pressure is at least 25 at a good hot idle, with a high quality 10w30, and the engine isn't making a racket, no reason to think it needs a thicker oil.