I don't own a W140 chassis, but I am sure MB transmissions are the same with regard to checking fluid levels. According to my owner's manuals, if you check the fluid when it's "cold", it will actually be below the "min" mark. "Cold" being you just started the car for the first time that day and let it idle for a couple of mintutes. Ideally it should be checked when the transmission is up to operating temp. Driving about 10 miles or so at least, should do the trick. You don't want to check it right after pulling off the highway though. Look at your owner's manual, it should say basicly the same thing.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section