My 2SZ-FE 1.3 vitz has a sticker inside its bonnet suggesting 0w-20 oil. To address that i was able to find Zic A+ 5w-20 however i experienced power loss and increased knocking in the car.
(Plz note that i had used Zic A+ in my Margallah for 3 years and 100K KMs so i know it is a very very good engine oil).
So i switched to Micking Fully Synthetic Max Premium 5w-50 API SM oil and have been using it for nearly year and a half now. It costs me 3200 Rs with the change interval of 5000 KMs (suitable combination of cost affectiveness and highest possible grade oil since Mobil 5w-50 costs around 5000 Rs).
Now the reason to choose 5w-50 was that the car was previously maintained by toyota ISB and they had been pouring in 20W-50 Toyota petron in it. Since the car has more than 120K KMs on the odo so i assumed that the clearance in the engine seals has increased as compared to the new engine so light weight oils like 5w-20 didnt allow the engine to make the expected compression so i experienced increase knocking.
Since 5w oils are usually more fuel efficient so i opted for 5w-50. 5w for fuel efficiency and better lubrication at cold start ups and 50 weight for hotter operating conditions and traffic jams in pakistan.
as a result knocking reduced by 90% and i am getting the fuel economy of around 12-15KM/L with AC in City.
Please remember that 5w-50 oil has higher viscosity index (better lubrication at higher temperatures) as compared to 5w-30 oil.
5w-30 oil should be cheaper than 5w-50 however to keep the maintenance cost low toyota says its ok to work with 5w-30.
my suggestion is to opt for shell ultra 5w-40 gray box API SN fully synthetic oil since you will be able to get it easily (micking is not readily available in market usually) and enjoy your ride. once it has been driven around 100K KMs you can increase the oil rating to 5w-50 viscosity.