To that regard get a new oil fill cap gasket and make sure the oil separator hose that leads to the top of the engine is tight and not leaking.
Also, check to make sure your pcv (positive crankcase ventilation) hoses and oil separator/trap are not clogged. If they are, replace them.

Smoke from the oil dip stick tube when removed right after cutting the engine off and the oil filler cap being blown up when allowing it to sit at the opening (not tightened on) when car is idling are both signs of a clogged pcv system.

You didn't say if you have a turbo or not, or if you did, I missed it. If your 850 isn't a turbo, take the flame trap out and trow it away.

All the clogging of vacuum/air recirculation systems can cause added pressure to seals and other lines...notably creating leaks. Sometimes doing this maintenance is enough to re-seal small leaks.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.