For me, it's worst case scenario. I tow 6,000 lbs with my 20 year old 5.3L with 270k miles. If I have to go up a steep grade for quite a ways, down in 3rd gear, lugging up the hill, I like to know the engine oil, ATF, and gear oil aren't going to miss a lick. I want what will handle the towing, not what will handle the easy daily commute down the highway. Any oil can handle the daily commute. That said, I still don't go to a higher oil grade. I stick with a 30 grade, but pick one with a >3.5 cP HTHS. The rear end is an exception where I run a 75w-110 instead of 75w-90.