What does the manufacturer of the car recommends? My mom's V40 T4 (2.0T) recommends anything from 0w-30 to 10w-60 depending on ambient temps.
To me, they key is that it needs to be a synthetic as it's less likely to break down and coke under the very high temperatures present in turbo applications. GC should be just fine I think.
For me, there is no other oil for a turbo than a syn and, I believe, GC is one of the best synthetics being marketed. I use it in my Audi 2.7T (twin turbo) and it performs superbly.
I just put GC in my 2004 V70R after two 5-6K OCIs with Amsoil 5w30. So far it has been great (same MPG as amsoil--no consumption). I agree with pscholte--high performance turbos like this should get only synthetic. One important thing: this is a small engine with a relatively small sump (5.5 qt) producing a lot of hp--which means significantly shorter OCI if you use one of the formulas for calculating this. In the real world I have found the TBN of Amsoil 5W30 to get a quite low by 5-6K miles. I would not try to make it the whole 7.5K OCI recommended in owners manual unless they are all highway miles and you are using a good synthetic.
How do you like your car so far? I love my V70R, its a blast to drive and I have had almost no problems in the past two years (knock on wood). Also the best forum for "R" cars is the "R" forum at Swedespeed (http://forums.swedespeed.com/zeroforum?id=51).