Hi,
I have personal experience with vitz 1krf, it runs best on 0w20, engine operates btw 100-120c, 0w20 holds till 200c, engine never reachs 200c, just change mindset with new technology n concepts , u won't face engine oil loss, unless yours piston oil rings have harden, keep running it, in case u observe 100-400ml oil loss after 3-4km.
0w20 is recommended by Toyota Japan where these vehicles originally coming from, visit website.
Rest consult with Raja Atta bahi, he will give best opinion