Lawnmowers use SAE 30 motor oil. The oil should be changed in a new mower after the first five hours of cutting. After that, the oil should be changed after 25 to 50 hours of use or at least once every summer.
Mowers need an oil change more often if they are used to cut wet grass or if they operate on rough ground or in dusty conditions. The oil should be checked before each mowing session and should be changed when it becomes dark. The oil filter, which keeps grass and dirt out of the oil, also needs to be changed once a year.