There are two possibilities for CAT brand hydraulics on your tractor, #44 or #41. The 41 was frequently used on toolbar setups which could be changed over from dozer to rear mounted toolbar. The 44 probably was the more common one and used on plain dozers etc.
If you look in between the radiator and the rear of the hydraulic tank, on one side or the other, just below the top of the tank, you should find a serial number tag riveted on. With that, someone can look up the filter number or you can call CAT and get one.
A #44 tank holds 3.5 gallons, but the whole system holds some additional amount in the lines and cylinders. On the #44, there is an internal filter which is held in by a 4 bolt cover on the bottom of the tank. There is also another beside it to drain the tank which is held on by two bolts.
Prepare for sticker shock from the filter price!
I would use at least 30wt if not 15W40 in the engine. 10wt is commonly recommended for hydraulic systems and CAT manuals from the era that the machine was made specify Crankcase Oil (CO). I would tend to use a tractor hy-tran fluid instead of a motor oil unless you just don't want to have two different oils on hand. Hydraulic service is a bit different from engine lube, so I believe you can do better with a fluid designed for that purpose. Case for example recommends using a multi-weight motor oil in their skidsteer loader hydraulics, but only with an additive special for that purpose.
Frequently when air is being entrained into the fluid, the pump gets noisy. Also could be showing signs of wear! A higher viscosity fluid could help for a while if the pump is worn, but will tend to operate slower than the proper weight fluid. Hydraulic fluids have antioxidation and antifoaming additives.
D2-5J's, D6-9U's, D318 and D333 power units, 12E-99E grader, 922B & 944A wheel loaders, D330C generator set, DW20 water tanker and a bunch of Jersey cows to take care of in my spare time😄
