Thick
oil if you race and turn high
oil temps and high rpm. It will give you the protection. If you have a street car and a 351 built with modern hydraulic or roller, lighter than what you are using could be better. It will take less time to get to proper temp and your car will feel faster. Drag guys use light weight
oil for a run on the street. The crank has to work harder to turn thick oil. If you are not running over 7000 rpm, I would go lighter. Check with your engine builder.