I found this online..maybe it will help
HST oil is changeable and does not require a technician. At the same time, the service manual does not describe how to do it and it is not obvious until you do a few things. This is what you need to do:
Step 1: Make sure you are looking at the HST and not the gearbox. The HST is the rear most transmission box and has the rear axles coming out of it.
Step 2: Take off the bottom rear protective cover. Take off the rear most bolts on the tractor; there are four bolts of them and the center two hold the tow bracket on. Take off the other two bolts that hold the cover on, they are located on the left and right sides of the cover.
Step 3: Jack up the rear of the tractor and support it safely. Then take off the right rear wheel. This will give you access to the HST oil level check hole.
Step 4: Find the HST oil drain plug on the bottom rear end of the HST. It uses a threaded plug with an Allen head (in think that a 6mm Allen head wrench works on it). Get an oil drain pan in position and remove the plug to drain the oil. Then replace the plug.
Step 5: Find the HST oil level check hole and remove the plug. The hole is on the left side of the HST, a bit forward of the left rear axle and is not easy to find. I had to lay on my back to see it. The plug looks very similar to the drain plug (it is probably exactly the same) and you can remove it with an Allen wrench.
Step 6: Remove the air vent line on the top of the HST. Mine has a black hose attached to it with a valve on the end. The black hose is hose clamped to a plastic fitting that threads into the top of the HST.
Step 7: Use a fluid pump (cost about $8 at an auto supply store) to pump the oil into the transmission. It should take about 3.7 quarts. The HST oil level check hole is a "fill to spill" type. So when oil drips out of the check hole, stop adding oil.
Step 8: Replace the plug, the air vent line, and the bottom rear protective cover. Your done.
This task is no more difficult than replacing the engine oil but might take more time since there are bolts to take off and replace.
Note of CAUTION: I did this to a TG1860 in an effort to fix a transmission problem that I was having that it DID NOT fix the problem. So if you transmission is not working properly, this oil change will not necessarily fix your problem.
