Congratulations on your purchase. In US, furst AWD's were in '98 V and S70's. I didn';t know there were any 850's AWD's anywhere. Where are you located? Anyway, My sense of oil consensus is that Dino is fine so long as you change it fre3quently, and there is no particular issue with changing to synthetic. I run Mobil 1 synthetic in everything most of the time, and have changed dino cars over to synthetic when they had as much as 170k miles on them, with no problems or leaks. I really like the ciold weather characteristics of synthetic, since it doesn't thicken up when it gets cold. This makes cold weather starts much faster, with less battery drain and much less internal engine wear at start. Something like 90% of engine wear is often reported to occur in the first couple of minutes of each use, and this alone makes the added cost seem OK. Also, there are a fair number of proponentw of slightly longer drain intervals with synthetic, say 5,000 miles versus 3,000 for dino. for teh oil leak, go to volvospeed.com, enter the Bay 13 section (repairs and maintenance tips and write-ups) and you will find lots of good tips, including one on teh turbo return leak and how to fix in detail. It is not a big deal. Good luck.
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Mike Sullivan ('91 745 (171k), '93 965 (147k), '95 855T (60k), '98 V70XC (78k). Past Volvo's: '85 744 (256k), '86 245 (165k), '86 245 (195k), '88 745 (208k).