Daryl, that is a tricky question. The general rule of thumb is to run the lowest viscosity possible that still delivers good oil pressure. Details about the engine should be considered… is it new with tight bearing clearances and good ring seal, or is it a high-milage engine with loose tolerances? What is the use of the engine… daily driver, high-rpm race engine, or other? I’d be sure to have a good accurate oil pressure gauge, then try to run a synthetic that delivers at least 20 psi at hot idle. Many would argue the old 10 psi per 1000 rpm rule, some even say that is higher than necessary. It’s all a matter of what you’re comfortable with. Use a good oil and change it often, that’s the key to longevity.