the recommended viscosity grade for vitz is 5w-30, using the correct grade is more important than the brand (as long as its genuine and reputable brand). 20W-50 is way too thick for vitz. you can use 0w-30 (zic), 5w-40 (shell hilux ultra, mobil1, zic xq (all are fully synthetic)). and if your engine is in good condition, you can safely use these oils for more than 10,000 km.
get your oil changed as soon as you can and you will definitely feel difference in engine responsiveness and fuel economy.