i changed the fluid and spin on filter (along with the hydro in line filter under the fender pan) before mowing season last year. a couple of weeks ago, i changed the hydro fluid again, as peace of mind. the oil was still clear. so, i will not changes it again for two years. i don't put very many hours on it per year. 33 hours last year. i was having to keep a close check on the hydro fluid level because i had a very small leak from somewhere. but when i changed the fluid the other day i noticed the o ring seal on the drain plug was a little distorted. i replaced the o ring and i believe that will fix that issue. also, when i serviced the transmission this year, i replaced both the suction and drain hoses as well.