Find out which Kohler engine you have (specific model number from the engine itself) and go to the Kohler website to download your free copy of the owner's manual and a free copy of the service manual for your Kohler.
Change the oil & oil filter every 50 hours, or once a year, whichever comes first.
Change the oil in autumn as your final action before storing it for the winter.
Check the engine oil BEFORE you use the machine each and every time you use it.