No gasket sealer is used, nor necessary. The seal is soft and pliable and easily nicked. Are you sure the pan is flat, not tweaked in any direction? Also, is the tranny is overfull, the fluid can come out the vent or other seals and look like the pan gasket is leaking.
Also, on both the 4.5 and the 560, I had to kind of milk the gasket to get a proper fit, then quickly get the pan up to the tranny before the gasket had a chance to move. Been in the 4.5 twice and the 560 once and no leaks from either.
Also, be sure you didn't accidentally mount the pan backwards. The 560 pan has one side taller than the other due to the way the tranny case is designed. If the bottom of the pan isn't parallel to the ground, chance are it is on backwards.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.