re: "...I converted my 300TE to synthetic at 68k miles (in '97) and abruptly blew my rear main crankshaft seal. The front cover seal and top cover seals went shortly thereafter. ..."
I really don't accept that any of those leaks and/or blown seals were the result of changing the oil. I've started using synthetic oil (always AmsOil 10W-40) in about 12 pre-owned cars over the past 28 years (7 Volvos of '70s and '80s vintage, four MBs of '80s to '90s vintage*, and one '85 Dodge GLH Turbo), and haven't encountered oil leaks that started after using this oil in any of them. Starting mileages (i.e., when I started using synth oil) were 56,000 miles minimun (one, a '80 Volvo 245, was started at 148,000). Sure, in some of them, at high mileage, a leak has started later, but these were many, many, tens of thousands of miles later (at high mileage), when it would have occurred anyway regardless of oil type. And, all the leaks were on Volvos -- none of my MBs leak yet (except for one with a head gasket leak that was fixed under Encore warranty).
[* because this is a MB list, I'll give the MB details:
'88 300E (M103), synth oil started at 124,000 (4 years ago) -- leaked a little when I bought it, and it doesn't leak any more than before at present 160,000.
'88 190E2.3 (M102), synth oil started at 88,000 (5 years ago) -- no leaks so far at 118,000.
'90 300E (M103), synth oil started at 56,000 (3 years ago) -- no leaks at all so far at 81,000.
'84 280GE (M110, double cam), synth oil started at 88,000 (10 mos ago) -- no leaks at all so far at 98,000.]
Hope this helped.