Changing your lawn mower oil every season is recommended by the manufacture to keep the engine in good condition.
You should always change the lawn mower oil every season or more often if you do a lot of mowing such as in commercial mowing but for general lawn mowing at your home you can usually get by with changing the oil every 2 seasons which is what I do.
My lawn mowers don't get enough use to warrant an oil change every season and the oil still looks good at the end of a season but I just stick to changing the lawn mower oil every 2 seasons and it's never been a problem for me.