Till now I've been using Castrol GTX 20W50 API SL. Car clocked 128K.
Can I go now for SM or SN grade?
Usage 20 km everyday congested city condition, occasional highway 300 km plus, AC on/off as per requirement.
Recently cleaned sludge out of valves here - https://www.pakwheels.com/forums/d-i...enating-santro
SN, SM all are downward compatible.
So yes, you can use API - SM or SN for cars that are run on older grades like SL, SJ, or SG etc.
by the way, for Hyundai Santro try 10w-40, instead of 20w. I hope you will like it.