the only reason it is specified is gas mileage, it isn't "required" (notice these are the very low gas mileage cars) And how many people here use what is actually specified anyway? I am pretty sure 20-50 isn't specified by any manufacturer.
Gas mileage is only part of it. There are also much tighter tolerances in these modern engine. I don't have the specs handy, but several manufactures have put out bullitens concerning what oil specs should be used. I do know that some of them will even trip a MFI light if the wrong oil is used (Chrysler).
