Folks, I haven't seen any guidence on GL-4 or GL-5 as the best choice for my diff. The manual also says that they prefer 90wt. or 85W-90, the latter I haven't seen. I there a problem using 80W-90wt? How about multi vis up to 140 wt? I would love to have the protection during the hot summer months charging up and down the mountains! Thoughts? Thanks Scott