As an Amsoil dealer I have made some very biased observations:

1) Synthetic oil doesn't cause leaks...as stated IF you already have a leak, synthetic oil will leak faster because if it's EXCELLENT flow properties. That's how the whole synthetic oil and leak thing started. Person has very small petroleum oil leak - put synthetic in, sudddendly notices leak. Best to start synthetic oil early.

2) Good synthetic oil, such as Amsoil is actually MUCH better than petroleum oil for seal life. Many multigrade petroleum oils contain pour point agents (solvent) to lower the pour point just to get the wider vis range...these solvents are hard on seals.

3) Petroleum oils break down at turbo temps into abrasive carbon compounds (really happens after solvents mentioned above boil off). This crap chews at the seal surfaces.

All in all good synthetic oil is better for seals than petroleum oil....and I'll even take my bias hat off for that statement.

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http://www.fidalgo.net/~brook4/oilslubesfilters.html

I just changed from reg to Syn oil at 75k miles. No leakes. I drive hard as well... I was worried about it too, but decided to go for the syn protection on the super hot turbos. I heared they can get red-hot. The car is better than before.
Hope i helped! :)

Chances are very slim (and if they leak they would have leaked sooner
or later anyway). Most common leaking seal reported here is rear main
seal and that can be caused by not servicing the flame trap (aka PCV
valve) annually (expensive to fix). The turbo return line also has a
high reported level of leaks when the gasket fails (cheap to fix).

Try google.com and type in "synthetic oil seals leaking"
to get a whole bunch of opinions on this topic.

Hard to say. In general, older cars tend to loak more with syn then with dino for a number of reasons. Seals swell and wear, gaskets get old, etc.

I just purchased a 97 855 R last fall with 70k miles. I run Mobil 1 5w-30 and I haven't noticed any leaks.

Give it a try and see how it does. clean the garage floor and put down some paper. If it does leak then you can find out where it's leaking from. It might be an easy fix, or it might be enough to switch you back to dino oil.

Personal opinion: it's worth a drop or 2 of oil to have the extra protection for my turbo.
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Nathan Valles, Black 855R, Tint