I also have a ZL2002DT and under stand the pain on getting any info,first off you have put diesel engine oil in your hydraulic system,the hydraulic oil filter is on the upper right side of the machine if your in the seat right next to the hydraulic pump,from what I have read your only suppose to use the sacred Kubota hydraulic filter,I'm running a Fram PH20A that is rated for hydraulics and Coop THF fluid,other equivalents are out there.
When you go to drain the oil again remember that you need to drain from,2 points on the transmission and the 2 lower points on the differential
I lucked out and found a VERY knowledgeable person in Saskatchewan,so far I have purchased all the rad hoses,New clutch,plate and bearing and also just recently ordered a photo copied shop manual,it might be in Japanese but Google translate is your friend.
Also the engine oil for this tractor is common 10W-30 as per the factory Japanese stickers under the engine hood.
For parts contact Sue @ Yellow Rose Ent,1 306 232 5570
Hope this helps