API SG grade is obsolete now, due to better technology available and is not recommended.
Going to upper grades only helps. Go to Wiki and search for API grading and you will get an idea.
Cultus is an old engine model thats why and at its time the grading was SG in the making. Now the world has moved on...
SAE grade is somewhat relating to viscosity, thickness, thermal requirement and heat resistance and behavior over temperatures etc. this is totally another game. Yes, 10W40 is what the Cultus engine loves the most in itself.
Just for an example the difference of SM and SN is that the later has lower amount of phosphorous (shayad) in its formulation/synthesis/blending. That gives better environment friendly gesture. And this formula was not possible/engineered when they were making SM grades.
Currently active grades are SL+ in Gasoline cars. And SM+ grade has some better additives in its formulation than compared to the SL one. So I will suggest to use any good SM+ grade oil. But SL is not that bad for Cultus requirement.
Just to add up also, in modern world you are not allowed to sell obsolete grading products at all. But here I am seeing that they are selling those as well as advertising also openly, robbing us with our precious earning with their pity products!
And that is the only thing why I always recommend to use imported and better brands. PSO, Shell, Caltex, Total, all are offering very inferior products currently as compared to what they offer worldwide. And also charging heavily for them!
