I have read posts about the pcv system causing oil leaks if it goes un-maintained. I have oil leaking at the rear of the oil pan seal. I was planning to replace the gasket and the pvc system. Is the oil pan removal difficult for the 850 turbo? Will I have to remove engine mounts etc to get clearance for all the screws around the pan? What other seals could possibly cause engine oil leaks or transmission oil leaks?

 

pictures will be helpful with the location of your leak.because l dont recall if 850s have a seal at the back of the oil pan. also YES you will have to remove the transmission torque mount to make enough room. also you will have to move the power steering line.there is a write up on matthews website on how to remove the oil pan to change the sump o-rings.to my knowledge there could be a transmission leak if your seals are bad and needs replacements also engine leaks can be cause by bad oil pan drain plug.mine is leaking from the drain plugs too so its time for replacement.
here is the write up
http://www.matthewsvolvosite.com/for...hp?f=1&t=31236

 

The PCV system on your Volvo provides crankcase ventilation and prevents the buildup of pressure from combustion gases getting past the rings and into the engine. If it's bad there definitely is the chance you'll have pressure inside the engine and it will push oil past seals.
One way to check it is to pull the dipstick up and check for smoke on a hot engine. There should be none or almost none. If it's smoking then it's likely a plugged PCV system. You can try to clean it out but from what I've read it's better to replace.
You can also try using an oil stop leak additive. They work to soften the seals. Seals tend to harden with age and have a difficult time sealing when they lose the flexibility to form to whatever they are trying to seal. If it's PCV related an additive likely won't do any good but if not it might buy you some time.