My rule is; If power runs through it, it wants thinner oil! The exception being a Model A transmission.The model A transmission has to be shifted and thick oil helps with the shifting and the sealing, aside from lubrication. I have tried Lucas Hub Oil, STP, and various concoctions, and like the engine, the model A is so forgiving and they all seem to work fine. While, I use a thinner rear end lube, too thick a lube kills HP. I tend to default to the heavier oils where shifting is involved.
Terry