Low Viscosity Oils: SAE 20W or SAE 10W?

The belief that low viscosity oils for motor vehicles will save considerable fuel consumption has now been proved to the satisfaction of almost everyone, and it is agreed that providing the engine is in good condition, very little extra wear will occur. In fact it is doubtful if extra wear will occur at all, but if it does, it is easily offset by fuel cost savings. The problem that has to be decided is how far can one go ? The use of SAE 20 oils is very common, but there seems little doubt that SAE 10W oils will save fuel. Many are advocating the use of SAE 5W oils.