Amazon has a great sale on the excellent Castrol Magnatec 0w20, $20 for a 5-quart jug free shipping with amazon prime membership. Name brand quality stuff.
Best 0w20 might just be BMW TwinPower 0w20, rated to LL14, BMW's tough oil spec which is tougher than dexos1. ebay or BMW dealership
Otherwise, go for Mobil 1 Extended Performance 0w20 at Walmart with a Fram Ultra oil filter, best filtering, silicon ADBV, synthetic media.