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-y-projects/211384-rejuvenating-santroSuggestions please.
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.
OK, so i would flush engine with 3M engine flush and thereafter use SM or SN grade oil.Preferably SM as there will be leftover mineral oil in engine and mixing with SM is ok.
Just use havoline synthetic. There is no problem in mixing synthetic with mineral