The motor oil in the Chetak lubricates both the engine and the "wet clutch." The owner's manual specifies 1 quart of SAE 20W40 (actually it only takes 9/10ths of a quart - any more gets sucked up through bleeder line into the air filter box) . SAE 20W40 is hard to find in the US. The former president of Bajajusa recommended 20W50 as a more common substitute. When filling the engine through the filler hole at the back of the engine (a squeeze "ketchup" bottle works well), simply ensure that the new oil makes it up and onto the threads of the filler hole. I have always used Castrol GTX auto oil available at Walmart but any good dinosaur oil should do. Many riders have tried synthetic oil and some, but not all, have had good results. The issue is the wet clutch - it needs lubrication but can't be too slick or the clutch will slip. Beware of "energy conserving" oils for the same reason. Motorcycle four stroke oils will often say on the back whether they are suitable for wet clutches.