Boy, are you opening up a huge debate. Every board I've ever been on, as soon as someone says "oil", there are two dozen opinions about which is the better practice, types of oil, brands, change intervals, you name it.

Anyway, I'll throw in my two cents. I've changed to fully synthetics (Mobil 1 and Amsoil are all I use) in cars that I was nearly certain the previous owner only used regular dino oil in, and had no problems, including the 850 I have now. The only car I have that I don't routinely use synthetics in is my Triumph TR6, because it leaks oil from a few spots and paying good money to put Mobil 1 in it is a waste. It's a British thing, they all leak oil at some point, most from the time they were new. I guess that's one good thing that companies like Ford have taught Jaguar, how to seal up a damn engine the right way. Anyway, I use extended change intervals with synthetics, usually 5K miles. I used to work for Mobil, and although I'm not a petroleum chemist myself (I'm was in environmental chemistry there) I saw some of the data that was produced during studies of oil breakdown under adverse loads and environmental conditions. Bottom line, top brand synthetics like Mobil 1 and Amsoil can easily handle 5K miles under normal day to day driving conditions. The guys at Amsoil claim some of their formulations can go well over 10K miles (I think their website claims something like 25K for their top of the line blend), don't know if I'd feel comfortable doing that though. I don't think Mobil advertises this fact though, they want you to change it every 3K and buy more of their product. The only time I would change a synthetic sooner than 5K would be if the motor was really stressed, maybe towing loads, if the car was run hard at the track regularly, or you're out west in the desert in the middle of summer, that kind of thing. If you going to stay with dino oil I don't think there's anything wrong with that, just get a good brand oil and change it regularly, and sooner than you would a synthetic. There's tons of data however that show that dino oil promotes the formation of deposits inside the motor much quicker than synthetics, so make sure you stick to the change schedule. An earlier post was correct though, the filter quality is as important as the oil quality. For the Volvo, I get the Mann filters, which I believe are OEM for alot of European makes. They are very high quality filters, and you can get great deals on them off Ebay, I get a case, ten filters, for 30 bucks and change. I don't see any sense in paying the rip off prices at the dealership for the same thing with the Volvo name stamped on it.

For some interseting insights and opinion on motor oils, change practices, and "exposing the 3 thousand mile change myth" check out the motor oil bible:

http://www.motoroilbible.com/