Welcome to JUST ANSWER heavy equipment. With years of experience, I look forward to doing my best to help. The older units, which include the 50, use a hytrans oil for the hydraulic system and the transmission rear end. I have seen people put 90 wt in them, but it is a bit too thick for cold weather and for the hydraulic system. You can use the New Holland spec. 134, which is a good hytrans oil. You can find an equililant at any tractor supply.
The filter is a screen that is in the three point area, and is not accessible without removing the three point assembly.
I would just drain system and start over with new fluid.
Shows to take about 6 gallons.
As far as a manual is concerned, I have never found one on the internet, I bough mine from an Ebay site, on CD.