I raised the seat and found a dipstick mounted to the flywheel housing between two (looks like) hydraulic pumps, right above there is a decal that says 80-90 gear oil. the dipstick shows its empty i dont see were the filler tube is or what its for.
There is a gear drivetrain powering the pumps and it is lubed by gear oil. Very likely to be filled through the dipstick tube, but frankly I don't remember for certain. An operator's manual would answer that question and many others that will come along.
As you are looking at your dipstick, look to the left and up above the hydrostat pump. There is a 3/4" street elle with a plug and breather in the end of it. Remove the plug and add oil through the street elle.