Just got a 2006 tigercat 630c skidder and was looking for where to add hydraulic fluid at
Most newer, dedicated forestry equipment will have a hand or electric pump with a filter inline to add or refill fluid to the system with clean, filtered oil. Look at the hydraulic tank and it should be easy to figure out if you have half a clue on hydraulic systems.
Our buncher had a pump and long suction hose. Could suck out of a drum without even taking it out of the truck.
From a 635D book I read it here https://www.scribd.com/document/663205194/635-d
I ran a 610c and it was identical to the post above.
Looks like a Fillrite FR152 hand pump. I had one on a slip tank but found a new electric pump on sale for a great price. Didn't realize they worked with oil too. Good pumps.