850 frame trap and oil leak

Synthetic oil won't cause less fumes, but will cause less deposit buildup. Cleaning out the throttle body may help. However, on the 850 the oil trap clogs up, and because of its nature it almost impossible to clean. Also the intake manifold needs to be removed to change it, it's a fairly long job.

Curious, cleaning up the flame trap cleared up the leak?

I replaced my rear seal in S70, on the theory that once this seal was breached, oil would keep leaking through - very costly repair

We have owned quite a few Volvos, and always have had them serviced at a local independent shop. The owner of the shop hires Volvo-trained mechanics. They have told us consistently to stay away from 10w-40 oil in all of our Volvos, from our 240 days to our V70. Mayhap there is a problem with one of the additives that make up the 10w-40?

Yes, the flame trap being clogged can most definately cause oil leaks. It makes pressure build up much higher than normal casuing leakage at the fill cap and other places. Left unchecked (or undetected) it can cause excess pressure on the rear main seal cuasing premature failure and necessitating replacement of it. (I went through this whole scenario over the course of 10 months on my 94 940. Of couse, the 920 only cost about $225 to replace the rear main.. not so nice on my wifes 95 850 when it was her turn for the same. I've hear it's very common for most Volvos to need a rear main at between 110-150K.