Guys and gals, keep it simple! If your car requires 5W-30, USE 5W-30. If it calls for 5W-20, USE 5W-20. If it allows 0W-20, 0W-20 it is!
Lower engine oil viscosity might provide an additional economy benefit, but if your car is not designed for a particular viscosity grade (higher or lower), expect bad things to happen and in the worst possible moment.
A lot of members here are using synthetic oils with excellent results. I personally use Mobil Synthetic 10W-30 (Toyota recommended for hot weather) but may use Royal Purple in the future; satisfied as it is. Mobil-1, Castrol, Eneos, Royal Purple, dyno or synthetic, USE THE RECOMMENDED GRADE.
*** The Toyota 2AZ-FE engine (I have a 2005) was updated to use 0W-20 in 2006
TSB Link.
Read the end section and note that they approved 5W-20 for a list of engines, but they allow 0W-20 SPECIFICALLY on the listed engines STARTING on Model Year (MY) 2006. I'm sticking with 10W30, especially since I drive the car hard!