I checked my owners manual on a model 1517 CD, it's a 17 hp Koehler, 42 inch cut. Says to never change the fluid, it's sealed for life. I suppose it has sealks and could leak. I sold a guy a pail of Conoco Hydraulic AW 68 for his mower hydra-stat recently, didn't quiz him much as he seemed to know what he wanted.