My point EXACTLY! I just had my dealer's mechanics change both HST filters and I took a close look at the fluid that came out of the filters and it was nearly perfectly clean- like new at 93 hours, no prior service of any kind on the tranny.
I also had them change the fuel filter; I had already done the 50 hr svc. on the oil and filter. That I used the Amsoil diesel oil for; which I also used with my earlier DK-35SE /HST.
The mechanics said they NEVER change the tranny oil at 50 hr service on HST's, and usually do it around 200-250 hours. SOMETIMES they do change the trans fluid on Shuttle Shifters due to those who don't come to a complete stop between forward and reverse sometimes causing premature fluid contamination. They did say that unless something fell in the fill hole with the cap/plug removed the chances of contaminating an HST were virtually unheard of- though occasionally some condensation can build up in the flexible hydraulic lines going to equipment like a backhoe through normal operation of the tractor and its equipment.
As to cars- my Toyota Forerunner does NOT have a transmission dipstick, and there is, ( I believe) no change interval- so its LIFEtime tranny fluid.
Some cars now don't even have OIL dipsticks!! The computer tells you when to drive to your dealer for the change of oil and filter.
So it is my personal opinion that HST tranny fluid change does NOT need to be done at 50 hours UNLESS there is visible contamination- milky colored fluid, or evidence of high levels of metal on the plug; which as stated above, and backed by my dealer's mechanics, is highly unlikely on an HST at 50 hours.
I always follow what the manual says regarding maintenance, the people who recommend the hour/service intervals must've had some researche done to determine this. That' why they probably earns more than me:laughing: I use shell donax TDL as hydraulic fluid in my DK45SE HST and my CK30.