All of the older B&S engines are supposed to use an SAE 30 engine oil that complies with the automotive test standards. In other words, multigrade oils are not recommended. They will not hurt the engine but it is likely to use more oil than it would with a single grade 30 weight oil. Remember, it still needs to comply with the automotive specification. Suitable oil is available from auto stores, but you may have to use a relatively expensive brand such as Penrite to get a single grade oil.
Remember to warm up the engine for a few minutes before draining the old oil, so it will run out freely and leave less residue.
Ordinarily four stroke engines such as yours do not require frequent spark plug changes (unlike two strokes, which do require frequent changes). Service the air cleaner, and make sure that the governor is working correctly by checking that with the engine not running, the throttle on the carburetor is closed when you set the speed control to minimum, and wide open when you set it to maximum. Inspect the engine for leaks/oily areas, missing fuel or oil caps, and verify that the recoil starter is working properly (including retracting, and not making a shrill scream when the engine runs). Check that the fuel is clean and clear, and there is no garbage laying on the bottom of the fuel tank. If there is any grass in the tank, tell us and we'll talk about how to clean the intake screen on the carburetor.
