I switched my '88 300TE to synthetic when it was 8 years old, and it definitively started to leak more. However, I'm a huge advocate of synthetics (my Honda with nearly 300k on the clock's never been apart, but when I replaced a rusting oil pan at around 260k I saw what appeared to be a brand-new engine on the inside) so I replaced the leaking seals (front crank seal, rear crank seal, front cover seal) and am convinced I won't have to again for a LONG time. However, when I tried out Red Line synthetic motor oil (which per the manufacturer is more of a "purist" synthetic than Mobil 1 and thereby lacks the additives that make your seals swell like dyno oil does) my reward was new leaks. Switched back to Mobil One; leaks gone. You decide what this means.
Now, as to the tranny issue broached by Chuck T., I was not aware that Mobil has ever produced a synthetic transmission fluid of a variety suited to a 928/MBZ auto tranny. Could that have been the problem? Please advise as to what specific product you used. I run Redline Synthetic ATF in both of my cars and it's been fabulous, and a marked improvement over conventional ATF in the cold (smooth shifts at very low ambient temps)and neither tranny leaks. My bro-in-law's run the same stuff in his '88 300TE from 100k miles to the present (just shy of 300k) and even though that tranny slipped when he got it, it never worsened and he got another 200k out of it! It's finally starting to really die, so he's popping an MBZ unit in this week, and will unquestionably run red Line Syn in it; he's now a convert and only runs synthetics.
I hope this helps.
Best regards, Michael