3 PT HYD

Here is a file I have on the unloading valve etc.

The 20 w oil probably would not have any effect on the operation of the lift unless it could have in some way caused the O ring on the unloading valve to change shape. The 134 oil is what I would use. However you guys who are where it goes down to 0 deg probably could use a lighter oil better.
I would first try bleeding the pump by removing the front pipe plug with the Allen head hole in it. Start the tractor and let it idle as slow as possible and when all the bubbles are gone stop the tractor and put the plug back in. You will loose about a half cup usually so put something under the pump to catch the oil.
If it won't work then you must go into the lift and take out the unloading valve and clean it up and replace the O ring with the Ford O ring made specifically for the unloading valve. The old part # was NCA 836B. Don't use a substitute or you will be sorry.
It is a good idea to replace the back pressure valve or at least see that it is working properly.@ has some resistance to air pressure etc .Also be sure the big O ring that seals the back pressure valve to the side of the lift is in good order.
The back pressure valve must be disengaged from the side of the lift housing inside by removing the large acorn nut on the right side of the lift housing and backing the large hollow screw out counter clock wise till a light resistance is felt. Stop unscrewing then as you will damage the valve if force is used past when you feel resistance.
The unloading valve is covered by a plate at the front of the control valve housing just above the control valve. The unloading valve is covered by a 25cent size plug.. The valve can then be pushed out with a punch of the appropriate size. By slightly tapping the end of it till the small plug pops out The O ring will be on the unloading valve that needs to be replaced.
If you will look around on the other reference pages of the N board etc you can probably find some good views of the lift to give you some idea of what you will be looking at.
When taking the lift control housing off the tractor remove only the bolts that are around the ;outside edge of the housing and do not remove any of the bolt that are inside this circle of bolts. If you encounter a bolt that has fine threads on it put it back where you took it out. The fine bolt hold the cylinder to the cover housing.
Good luck Zane in Al.