It's entirely possible that you may be able to have sam walton's
heir's conglomerate safely and efficiently change your oil... but..
To reinstall tire/rim assemblys "wm" uses no torque wrench
(just the "Torque to lbs. inserts" for an air hammer wrench).

I know of "wm" automotive employees with A/P (airframe/
powerplant certifications);but then I have seen other employees, whose ability to check tire air pressure, I would have trouble believing.

(no I can't explain how or why anyone with an A+P license
is rotating tires at "wm".)

The question would be does your S-420 get it's oil changed
by the person, who has it's best mechanical interests as
primary objective?

I'll be honest with you I would not let the DEALER in the
city where I lived change the oil on a 300 SD I owned !

You can SAFELY change your own oil (research proceedure first)or with due diligence find a reputable independant. OR
blow the big bucks and visit an authorized M.B. dealer