I'm a bit confused. Are you saying the viscosity of oil you run may affect the condition of your valve cover gasket?? I strongly disagree.
As
@warp9.9 stated earlier, the thicker viscosity helps the tranny a lot. Usually thicker is better for gears (to a point). That's one of the reasons we run a multi grade in our bikes (along with making cold weather starts easier). Oil is used by the engine and tranny. The viscosity is a compromise. With automotive car engines don't need to worry about the shearing factor gears present so they use a lower viscosity even down to a 0W20 in some instances giving better gas mileage. We run a 40 or 50 weight mainly to help deal with friction in the tranny which really has no effect on gaskets.