The trans in my SDL does this. If the car sits for a long time the front seal which I have replaced BTW will start to leak and leave a nice puddle. If I drive the car its dry as a bone.

Depending on how much its leaking I would just ignore it. If its dumping out 3-4 quarts I'd fix it though. You need to drop the trans to get at them, which on the W140 doesn't look like to huge of a job.

Edit: just saw the person who brought it back to the top has a 722.6, fix it. That trans doesn't have a built in dipstick so keeping an eye on the fluid is a PITA. I read the first post I think the S600 has a 722.3.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200