When I got my other DGT 1700 I took the deck off, cleaned, oiled, greased and changed the gear box oil and belt. Upon using the deck I found some oil leaking but was not sure from where. So I removed the deck, ordered new seals, broke open the gear box and changed the seals and refilled the gear box up to the level of the fill plug (about 2/3 full)
The vent is about 1/2" above the fill plug. When I mowed with it on Sunday I stopped after 3 minutes and no leaks. I mowed for another 10 minutes and stopped. I saw some oil dripping somewhere off of the gear box. I wiped it all up and ran again with some leakage. I suspected that maybe it was coming from the vent. So as a test I cleaned up all the oil and parked it overnight.
Last night I looked and no oil leaks at all. So I ran the mower deck last night until the gear box got hot and no leaks. I drove it around and mowed with it again and no leaks. As an experiment I drove it up the steep hill on the front yard where I mow then I parked it on level ground. There I saw oil that had come out the vent on the back of the gear box and dripped down onto the deck.
So as I suspected, I over filled it a little and it was just running out the vent when the deck was on a steep angle. So in the end I replaced two oil seals that never needed replacement. I wish I could have found the recommended oil capacity for that gear box. It would have saved me two weeks down time and a lot of work. I thought I would pass this info along to everyone should you see oil leakage on one of these gear boxes after changing the oil. Live and learn.