Owner's manual specifies a transmission fluid and filter CHANGE, NOT A FLUSH, at 80,00 miles, with no further service nor even checking of fluid level. Some argue that a flush might dislodge debris accumulated into firm globs, which can then migrate to small control orifices, maybe jamb valves but a drain without flush likely allows these globs to remain in place., Cap of dipstick tube to be reinstalled. I DO NOT KNOW IF THIS CAP IS TO MAKE AN AIR-TIGHT SEAL WITH THE TUBE. Sealed-for-life automatic transmissions and hydrostatic drives are common these days. I have a sealed transmission on a hydrostat lawnmower going strong after 20-plus years of use.