If the VTIZ engine is 1KR-FE, and car is less then 80K driven and engine is in good condition, then try to use 0w-20 oil. Yesterday I read the manual and it clearly says to feed the following oils
1000 CC engine (1KR-FE) 0W-20
1300 CC engine (1NR-FE) 5W-30
They repeatedly mentioned in the manual that for "BEST FUEL CONSUMPTION" feed 1KR-FE with 0/20 oil. it will give the best mileage.
Presently Kixx G1 (Tin pack) is 0/20. Price 2800/4L. SCI is 5~6K. Just forget either its summar or winter, if you want to save on fuel consumption, then 0/20 is the best choice.
Moreover I've also discussed with LM's head office Germany. The range of oil viscosity for 1KR-FE engine is from 0/20 to 5/40.
It means these engines will not work "ideally" if we will feed them 10/30, 10/40, 15/40, 20/50 etc. oils.
I'll post this information as separate thread also so that more people can read it.