One of the members on our Aussie forum just took his 420SE on a long trip. He has done the same trip in his 300SE. The 420 gets better Gas Mileage. - One reason being the Diff ratio.
My own experience is that an old V8 380SE gets better gas mileage than a straight 6 280SE

if you think that the W126 300 3 litre was under powered - in the ROW there was a 260SE - 2.6 litre M103 W126.