Clevy, that was a typo I meant Mobil 1 Advanced Fuel Economy 0w20 not Mobil 1 Advanced Fuel Economy 0w20 as I posted earlier.
About the viscosity I was wrong though. Unless Mobil changed their data sheets again the Viscosity, @ 100ºC, cSt (ASTM D445) for Mobil 1 High Mileage 5w20 and Mobil 1 Advanced Fuel Economy 0w20 are nearly identical now at 8.6 cSt vs 8.7 cSt respectively.
I thought high mileage was near the upper edge of 20 weight range whereas the 20 weight viscosity specs are required to be virgin less than 9.3 cSt.
Someone experimenting with stepping down a grade might not want to go from 30 weight with a Viscosity, @ 100ºC being 12.4 cSt to a 20 weight with a viscosity at 5.7 cSt, which is technically possible based on SAE allowable ranges. That might be too far a jump in oil pressure.