Toyota has approved the use of 5W20 GF-4 oils (e.g., Mobil1) in most modern engines, and 0W-20 in 2AZ-FE engines as shown in the folowing TSB.
I have been using the 5W-20 oil in my 5VZ-FE 4Runner engine for about 6 months now and can see the difference in mileage.
It doesn't appear that either of these oils are approved for the 22R, 3S-FE or 5S-FE engine. I have been using a 50:50 mix of the 5W-20 and 5W-30 Mobil1 in my '96 5S-FE Camry for about 8k miles without any noticable problems. I have also notices a marked increase in mileage in this engine. I have been getting 410 miles/tank (about 27 mpg with 75:25 highway:city). I used to get around 360-375 miles per tank.
