For new lawn mowers,
change oil after the first 5 hours of use.
For continuous use of lawn mower, ideally, change the oil every 25 hours.
For regular maintenance, do an oil change at the start of the mowing season.
For old lawn mowers, always check the oil after a day’s work with a dipstick and change the oil when it gets murky.
Old lawn mowers’ engines usually have a lot of carbon deposits and some of it enters the crankshaft and mix with the oil.