Assalamualikum Brother. Indus pours and uses and recommends the 20W-50 Mineral Petron Oil because of its all season all Pakistan support. The Oil can be used everywhere and is readily available everywhere in the shops aka 3S Toyota showrooms/company operated service areas. Company policy dictates to use simple oils. The reason why they dont recommend it depends on many reasons including all region and season support, warranty reasons, it may not be suitable for one climate or dust conditions. Indus ki marzi hai woh kaun sa oil recommend karain, app ki marzi aap ki gari hai aap kiya dulwayeen.
Mineral Oils and Synthetic Oils ka difference aap internet pay dekh lain. I would suggest getting any fuel saving, fuel economy enhancing 10W-30/40 or 5W-30/40 motor oil which is semisynthetic or fully synthetic for a longer oil change interval and fuel economy. Baki your car, your rules, your choice, your responsibility. Peace Out! Jazakallah.