I am not nearly as educated on the why X is better than Y but I've towed quite a lot and a 40 is always going to hold up better than a 30 when doing that. When you are maxing out payload and towing, I'm going 5w-40 all day. It's crazy how much additional strain is placed on your powertrain when towing even light, and I say 3200 is on the light side of medium.
