The first a most important thing.
Oil is much cheaper that a new motor.
Also be careful not to overfill your oil. A guy gave me a lawnmower, a while back, and he said the motor was shot because it was burning oil. Use sae 30 oil in the summer. This will help the motor stay properly lubricated when it is hot. Use a multigrade, such as 10w30, in the cold so the motor has proper lubrication at start up.