I know this is an older post, but my 1520 did have this problem (oil leaking from the bolt head with the cotter pin) back in 2013 & 2014. It started as a drip and over about a year, it slowly got worse, then one day it got a lot worse. I split the tractor, removed the hydrostat pump/motor assembly, and took it to a hydraulics repair shop where they rebuilt it. I then reassembled the tractor. That was in 2014 and 2400 hours. Tractor now has 3600 hours and it has been dry ever since. I did replace all the o-rings on all hydraulic fittings that I had to remove.
At the time, the shop owner told me it was in really good condition except for a bad seal. He did recommend that they replace the input shaft if I planned on keeping the tractor.
The tractor has had CNH oil in it since new (with a one time exception of UDT) with frequent oil and filter changes.
Bottom line, if you have oil dripping out of the bolt head with the cotter pin, and it is not black diesel motor oil, you have either a leaking o-ring on one of the hydraulic lines connecting to the hydrostat, or you more likely have a leaking seal on the hydrostat unit.
And speaking of the bolt with the cotter pin,,, it is good to grab the cotter pin and spin it around a few times every now and then. That is intended as a drain and it can get plugged with dirt and not drain. Water can then collect in there, especially if you like to power wash your tractor.
