I understand that lower vis. oils provide less friction and therefore more MPG. I think what would be more ideal, but costly is to make 30wt viscosity oils more friction modified. Something like Redline or S2k. Amsoil claims their 5w/10w-30's exceed OEM fuel efficient specs. I bet Red Line 5w30 has less friction and better gas mileage then some of these Xw-20 oils out their. Doing it this way ensure the best of both worlds.
If you notice in 3MP's study, the Amsoil as it thickend made the engine more sluggish. This is what Redline found when testing Amsoil S2k. Amsoil being mostly all PAO thickens over long drain intervals more so then other synthetics, or so it seems. As it does, you loose that MPG and HP. When 3MP went back to Mobil 1, he noticed how much better the car felt.
Biggest reason for me not to run Amsoil was if I'm starting with one of their 12cSt oils, and it becomes a 14cst, I'm now in the 40wt range. With Mobil 1, starting at 10.5cSt and a HT/HS of 3.2, I have more room for thickenning and less drag on the engine over a longer drain interval. Mobil formulates this way for fuel efficiency but it plays to their advantage in long drains IMO. The 0w-40 in a 20,000 mile drain would end up right around where it started or even higher I'd guess.
If you notice in 3MP's study, the Amsoil as it thickend made the engine more sluggish. This is what Redline found when testing Amsoil S2k. Amsoil being mostly all PAO thickens over long drain intervals more so then other synthetics, or so it seems. As it does, you loose that MPG and HP. When 3MP went back to Mobil 1, he noticed how much better the car felt.
Biggest reason for me not to run Amsoil was if I'm starting with one of their 12cSt oils, and it becomes a 14cst, I'm now in the 40wt range. With Mobil 1, starting at 10.5cSt and a HT/HS of 3.2, I have more room for thickenning and less drag on the engine over a longer drain interval. Mobil formulates this way for fuel efficiency but it plays to their advantage in long drains IMO. The 0w-40 in a 20,000 mile drain would end up right around where it started or even higher I'd guess.