I used it in the summers in Denver, CO, and here in The Swamp in my 420SEL. The gas mileage was nothing to crow about, but then it never was. The 20W-50 is fairly thick, so you might see a mileage hit.

I did notice that my hot oil pressure on the gauge read a bit lower with the 20W-50 than with the gas/diesel rated Chevron Delo 15W-40 I switched to during the W126's last years. Not a lot -- 1.6 bar (I'm estimating) vs. 2.0. (The brand of 20W-50, Valvoline, might have had something to do with it, since there's variation in hot viscosity among brands, even within the identical grade.)

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-- Paul W. (The Benzadmiral)
('03 Buick Park Avenue, charcoal/cream)
Formerly:
'97 C230, smoke silver/parchment; '86 420SEL, anthracite/light grey; '84 280CE (W123), dark blue/palomino