First and formost, I'm all for 5w-20 and 0w-20 oils. I'll defend them all day and all night...BUT not in this case.
I tow 1000 lbs consistantly during the summer with my little 4cy 2004 Accord. I posted a used oil analysis not long ago with 6300 miles on Valvoline Syn 5w-20, and it looked amazing.
What leaves questions is, "real" towing vehicals call out heavier oils. If you tow often, IMO there's absolutly no reason to run 5w-20. It's designed for to increase MPG's, nothing more. And when towing...MPG's isnt your biggest concern.
I asked this question to Mobil and Valvoline. Each said the exact same thing. Occasional use was fine, but if towing consistantly, seek the advice of your local dealer. LOL, well, I'm not about to ask my dealer. But if you assume they dont want to specifically state to use 5w-20 for towing...you should also question it.
Perdsonally, if I towed any more wieght than I do now, or any more often, there's no way I'd keep using 5w-20.
And I'll come right out to say I have no basis for this. I just do not believe 5w-20 is for hard working engines. And towing 7-8k is work!!! Why not use 5w30 or 5w-40 to make you feel all warm and fuzzy?