[W126 Coupe] Oil leak - transmission advice

[stefansj at canisius.edu]

Hey everyone,
I recently changed my oil with Mobil 1 0w-40 (My first synthetic switch). The 
engine runs wonderfully, however, I have now encountered another problem. 
The car sat for 10-15 years before it came into my posession developing a 
minor leak between the tranmission and engine. This was very minor until I 
changed over to the Mobil 1. I assume the synthetic change really cleaned out 
the seal making the leak a concern. I plan on taking out the transmission and 
replacing the seal (I know, a pretty big job). My tranny has always had slight 
slippage until it warms up (after accelerating 3-4 times). So my question is, Is 
there anything I can do while I'm in there to fix the slipping? Anything I should 
replace since Ive gone that far? The car is a 83' euro 380SEC with 78,000 miles. 
Any advice you guys could give me before I take on this job would be a greatly 
appreciated! thanks, Jas