Following is my opinion:
Bro as far as I can understand, owners / users manual for Vitz does not contain any specific mentioning of 100% Fully Synthetic or Synthetic, Semi-Synthetic or Mineral, etc. and Toyota here can not restrict to any of these.
We all know; 100% Fully Synthetic or Synthetic is always better than Semi-Synthetic, is always better than Mineral. Reason: Their composition, uniform properties, molecular formulas and formation, additives, cleaning / detergent properties, etc. (Fully synthetic deteriorates after comparatively long time, mineral based oils deteriorate quite early when compared to fully synthetics). Toyota here can only guide to follow a specific grade; like 5w-30, meeting latest API SN / SN Plus, and / or ILSAC GF-5 rating.
Definitely, you can find 5w-30 meeting latest API SN / SN Plus, and / or ILSAC GF-5 rating, in all 100% Fully Synthetic or Synthetic, Semi-Synthetic or Mineral.
Let me give example of each, and going no further than Totachi:
Ultima EcoDrive L 5W-30 and Ultima Ecodrive F 5W-30 are 100% Fully Synthetic (Source: https://totachi.com/product/ultima-ecodrive-l-5w-30/ and https://totachi.com/product/ultima-ecodrive-f-5w-30/ )
Eco Gasoline 5W-30 is Semi-Synthetic (Source: https://totachi.com/product/eco-gasoline-5w-30/ )
Fine Gasoline 5W-30 is pure mineral based (Source: https://totachi.com/product/fine-gasoline-5w-30/ is based on a bit old API / ILSAC rating but https://www.penriteoil.com.au/applications/car-4wd/engine-oils/premium-mineral-5w-30#/ and https://www.penriteoil.com.au/applications/car-4wd/engine-oils/vantage-premium-mineral-5w-30#/ are mineral based, meeting latest API / ILSAC criteria)
Mineral will require frequent and early change intervals, but then of course, its price will be the cheapest in all the 3 categories. Semi-synthetic will be a bit expensive and require comparatively long change intervals. 100% Fully Synthetic / Synthetic will be the expensive in all these 03, and will have the longest drain intervals when compared to these other 02 categories.
Cost to benefit ratio may be the same, however, its also the additives which matter the most!
(The best of the best are added in fully synthetics)
IMHO, one should always prefer and go for 100% Fully Synthetic / Synthetic oils, in the best health and interest of vehicles.
I hope, it all helps. Thank you.
(If further details are required regrading comparison of these 03 and which would be best suited, one can Google)