I have tried using Shell Helix 20 w 50 engine oil in one of my cars ( Santro Zing Petrol) and fouond it to be almost at par with Castrol 20 w 50 engine oil . There is a difference of about 10 % in the MRP, and the discount is higher so the market-operating price of Shell Helix is about 22% lower than Castrol ( non systhetic) Looks to be a decent saving