I have always used the recommeded oil in the trannys per manufacturers recommendation..not so much that the tranny will not get lubed, far from that. The lighter oil will lube very well, even better because of vicosity..but the difference in the oil is also rated in the SHEAR factor, ability to stay on the wear surface under extreme pressure thus ensuring that the wearing surface is not put in position to accelerated wear. Granted some of the newer manual transmissions do use these lighter oil but then again, they were built with this in mind. Just my opinion...