modern oils have so many additives in already you could end up doing more damage than good, back in the dark ages when all that was available was plain mineral oil i would have said go for it, but now there is no need, you could make a very slight improvement, but on the other hand you have agreater chance of reducing your engine/gearbox life
the best thing you can do is put the best oil you can in and when it comes time for a change flush the engine with either a wynns or forte engine flush