don't fear the 20w oils. I even use it in my jeep which sees towing and some trail thrashing and have had no reason to worry about the oil weight.
Mobil 1 is always controversial here. in years past (don't know about now) it was branded with having shearing problems, which was harder to debate/argue/prove since it seemed to vary based on it's grade. I personally found it would shear sooner than other synthetics when pushed to 10,000 mile intervals, and suspected that the 5-30 would shear much quicker than the 10-30.
The 20wt stood up will in my honda 2.4L, though PP and castrol stayed more consistent in terms of noise/feel over the full 7,500 interval.
My honda mower /hated/ mobil 5-30 and always had sparkles in the oil until I switched to something else.
I still run mobil driveline fluids and have nothing but good things to say there. But in the engine, forget it.
M