The mower will fall apart long before the engine does. Do yourself a favor and buy an hour meter for the engine. They are invaluable for knowing when to change the oil and filter. Hour meters can usually be purchased where commercial mowers are sold.
Kohler Command engines normally need the oil changed at 100 hours and the filter at 200 hours (factory recommeded) although a lot of people cut those intervals in half including me. You new mower should come with a very nice manual for the Kohler engine that explains all of the maintenance requirements in detail.
You can put Mobil 1 synthetic oil in the engine after the first oil change.