Do you have a turbo? If so, then full synthetic is the best choice. Otherwise, it's one half dozen or the other...just change the oil every 3000 miles as recommended. If you have full synthetic (ie: Mobil-1) you can go twice that.
As far as viscosity, I go with 10W-30 in the fall and winter and when it starts to warm up outside in mid to late spring, I switch to 10W-40, but that depends on where you live too...I live in S.E. U.S. Most all information of this nature is in your owners' manual.

I'm sure there are all different opinions here in regards to oil maintenance including the guy (Breadboy it is?) with almost 400,000 on his 850 who's oil usage and type, etc., seems to have been haphazard during the life of his car. Main thing is to keep the oil changed before it has a chance to break down.

My S70 (I assume it's the same quantity as your 850 but check your owners' manual) is just over 6 quarts (without draining the oil cooler which is a pain and not much comes out anyway so I leave it alone), so tighten the oil pan nut and oil filter and then fill with 6, check everything and then top off as needed.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon