If you have a slightly leaky transmission, then I wouldn't use synthetic in it. The reason is that synthetic is a 'thinner' oil, and will more than likely make your leaks worse. I would simply use a good dino trans fluid.
Also, if you don't know when the fluid was last changed, I have heard that you should change it and drive it for 1 or 2 thousand miles, and then change it again, because this will dislodge all of the sludge and so on. Good luck.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)