You might want to ask one of the major oil companies.

   My experience - and that of quite a few other people - is
that synthetic oil doesn't harm new rubber parts, but when you
switch to it in an engine with old seals that were used with
standard oil, you are rolling the dice. Japanese cars seem to be
less likely to develop leaks than Volvos, and not all Volvos will
develop leaks, but unless you drive an early 850 (these had oil
distribution and lifter problems that synthetic oil alleviates),
it really isn't, IMO, worth the risk of switching. Volvo engines,
especially the "red block" fours, will do just fine on standard oil.

   If you just bought a new Volvo, by all means use synthetic.

Go here, and check out the forums.

http://www.bobistheoilguy.com/

None better for your type of question.

This is the last word.... No, but if you have seals that are sealing
because of a build up of crud, the synthetic might clean out the crud
and you'll leak