first off when you take out the old oil you have to make sure that it is properly disposed because our earth is already bad, then look online for the proper oil in google, purchase and refill and your set
Most lawn mowers use 30 weight oil. You need to check your owners manual to be sure of the correct oil weight and type. You could substitute other oils if you work working in extremely cold temperatures. For example: If you where working in below 20 degrees, you might use a 5W-20 or 5W-30.