In Reply to: Re: oil weight posted by Lee on September 09, 1999 at 06:59:24:
You are quite right, 50 weight oil is thicker than 40 weight oil. But the actual viscosity depends upon the actual oil temperature (at the bearings, piston ring face, cams, rockers, etc) not the sump temperature. As oil heats up it gets much thinner, so that a hot 50 weight oil may be considerably thinner than a cooler 40 weight. Heavy weight oils also create more friction and heat. They do however, keep metal surfaces from touching each other. The SAE oil viscosity designations are merely nominal values and do not reflect the actual viscosity of the oil. Every oil contains about 25-30% additives regardless of the basestock. Use a heavy weight oil for high temperature engines, low speed, heavily loaded (lugged) engines. Use a lighter weight oil for properly cooled, high speed engines. some Indy cars run 5 (yes five) weight oil. As a general rule use the lightest weight oil you can, with the best additive (anti-wear)package you can find. Change the oil and the filter often.