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I think 10-30 would be the correct weight oil with stock bearing clearing clearances. Oil Pressure should be 10/15 lbs at idle and 40/50 at cruise. The reason is the lighter oil cools the bearing better than the heavier oils. However, the heavy oils make the driver and oil pressure gauge happier. Now the factory made industrial engines, that had a tag fitted to the block that stated the bearing clearance. Yes an old engine might need a heavier oil