what I see from this, is the wd-40 is cleaning out any grease or oil and debris that's in there, and the soap and water remove the wd-40 and leave a light soap residue that sorta kinda lubes the tumbler.
I'm sure it would work like this, but getting ALL the wd40 out with contact cleaning spray and installing the MB correct tumbler lube sounds like a better solution.

I've never heard graphite was an abrasive, but it's GOTTA be some sort of abrasive, even water is an abrasive due to it's oxidation properties. remember, some kinds of tumblers are softer than others, so while graphite may be fine for a house door tumbler or a chevy, it's likely not correct for a German car. it may work as a stopgap, but it may also cause other problems. especially if there is a lubricant in there, it tends to gunk up, and clump causing the key not to fit right.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!