I used to use 5W-20W Mobil 1 synthetic in our Ford V10 motorhome, but I switched to Mobil 1 Extended Performance synthetic some time ago.
Even though I now use the Extended Performance version of Mobil 1 synthetic, I still change it at the recommended stock oil change intervals. I figure that Extended Performance Mobil 1 synthetic may have even a superior formulation to regular Mobil 1 synthetic, so I'm taking no chances with our RV's engine ... it's too important. The cost difference is trivial, since I now buy my Mobil 1 Extended Performance EP 5W-20W synthetic at Walmart in the super-economical five quart container size.