I used M1 5-20 in an engine calling for 10-40 in 1978. Engine started much better in cold temps and ran smoother as well in all temps.
AND we've been over (and over) that the M1 5W20 was VII free, all basestock, and was more like Redline 5W20 n reality, with it's 2.9HTHS (which some would call "really a 30, including Dr Haas and CATERHAM, in spite of what J300 states)...and that was in an Era when 10W40 had no HTHS minimum, and may of them were woefuly inadequate....which lead to some OEMs decrying 10W40, and denying warranty should such oil be used.
Industry then started investigating "apparent" viscosity in high shear, and assigned 0W, 5W, and 10W 40s the same HTHS minimum of 2.9 as they assigned the 30s...that's how "good" the 10W40s were back then.
And so, when Mobil stated that their 5W20 monograde (no VIIs) gave the same protection as a 10W40, they weren't telling fibs...but they aren't (and we've been over this more than a dozen times) the same measures...and TGMO has nearly nothing to do with M1 5W20 the way it was originally made, in any way that you cut it....