Clevy, that was a typo I meant 0w20 AFE not 5w20 afe 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 M1 HM 5w20 and M1 0w20 AFE 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.