SJ10W-40 is the type of engine oil required for gas-powered 49cc scooters. It is recommend that you change the engine oil every 300 to 360 miles.
If you purchased your scooter brand new, it is recommended that you not only change your engine oil, but your gear oil as well. This should be completed after your initial 90 miles has been driven on the scooter. After your initial oil change, you should then stick to the aforementioned 300- to 360-mile average before performing an oil change.
SJ10W-40 is a common oil and should be easily found at any automotive shop or discount warehouse store.