Valvoline's Australian web site recommends 20W-50 in our 2U-ZE engine. I have very good results with 15W-40 in my Tundra in the summer here. You could use either Mobil 1 10W-30 or 5W-40 with 10,000 mile oil drain intervals with very good results. If you're still under the warranty, stick with the longest drain interval shown in the manual.
http://www.valvoline.com.au/servprodguide.cfm
Ken