I just returned from towing 5K pound trailer cross country and back (10K miles) including high peaks in the Rockies. I added an after market transmission cooler before leaving which seemed to work well; transmission temps sat around 195F for hours on end, never exceeding 199F even on long climbs. The Yukon now has 20K miles on it so is still pretty new. Should I change out the transmission fluid as a safety factor or was this adventure considered light towing and there is no need to change the fluid?