A Honda Rebel or CMX 250 motorcycle can use a petroleum-based or synthetic motor oil that is additive-free, such as lighter-weight 10W40. Grades of motor oil are based on the oil's viscosity and whether it has been tested for cold-weather use.
When a "W" is included in a motor oil's rating, it indicates that the viscosity of the oil has been tested for cold weather. Motor oil ratings that do not feature the "W" have only been tested at an engine's average operating temperature. The temperature falls at 100 degrees on the Centigrade scale or 210 degrees Fahrenheit. An oil with a lower viscosity rating means less wear-and-tear on a motorcycle's engine over time.
The oil should be changed for the Honda Rebel 250 every 4,000 miles. An oil change should include replacing the spark plugs and air filter and inspecting and adjusting the valve.