In support of what Ian mentions, a 10W-40 oil by one manufacturer may not be comparable to the 10W-40 oil from another simply due to the spread of the ranges used in the viscosity measuring process. Differences in the actual testing procedures can also account for some of these differences. As a general rule, warmer oil always makes more horsepower simply by lieu of the oil viscosity being lower than the same oil at a cooler temperature.