I was facing the same problem... Toyota guy put 20w50 in my vitz 2008 1.3.
The drive was heavy, and mileage around 11 to 11.5
I went to magic motors cleaned the fuel injectors o2 sensor throttle body changed the spark plugs....
Then went to toyota motors... and put 10w30 petron plus in the car... Is this oil good enough... I don't know how much my ride is driven.
Shall update you regarding the mileage in 15 days.