I've been using both GC and M1 0W20 in my Mazda 3 with the 2.0L. The GC has been the summer oil so far, the M1 the winter oil. I'm not totally convinced of the benefits of xW20 oils yet, but since it is so cold here in the winters, and the car does call for it, I've been going for that because of the better startup in the winter. I can't vouch for what seems to work better, as it's pretty hard to determine since I haven't done a UOA yet. The thicker GC does seem to affect fuel economy somewhat. Probably will try Esso XD3 next, or Redline 5W20, if I can find it!
Mike
Mike