1954 Ferguson transmission/hydraulics

Had to replace the seal on the PTO shaft on the old girl. Lost a good deal on fluid in the process.
Is there a recommended lubricant for this tractors transmission/PTO and do both units share lubricants?

Is there any way to flush these units as I suspect that the fluids might be original from 1954.

There are 3 drain plugs under the unit one plug for transmission, one for hydraulics and one for differential. It left the factory with GL-1 80/90 weight oil. Many have been using Motor oil especially those in the colder climates. There is a lot of good reading in the following link. Jeff-Oh explains it in detail. While you are under there check the cotter pins to make sure they are able to turn around in the holes under the clutch housing.
cleaning HYD., Trans. and diff.

Anytime you ask a oil question you normally get a long list of opinions,

I have a well used 1957 TO35 (MHF) the transmission hyd oil that came in it was GL-1 90 weight mineral oil non detergent oil
Reason I was told they used non detergent oil back then was the transmission hydraulic system does not have a filter, therefore the dirt
particles had to fall to the bottom of the sump and were to be taken out as you changed the oil,
The new tractors have oil filters on their transmission hydraulic systems so they use a detergent oil that keeps the dirt particles suspended in
the oil, the dirt is taken out as it passes through the filter,

Also the bearings in the transmission are brass or bronze (yellow metal) these new detergent oils can have additives that can attack the yellow
metal of these bearings,

All this is to say, if the transmission and hydraulic system are working properly, why not take the old tractor out and work her hard,
Now the transmission oil is as hot as it's going to get, pull up in the shade and drain the oil, it will take a long time for it to all drain out,

Just add the required oil, you can buy GL-1 90 weight non detergent oil at Napa they may have to order it but they can get it,
Change your oil and motor on

Just my opinion, you put some type heavyweight cleaner in that sump and it starts breaking down the gunk that has built up over the years and
you start pumping that crap through your hydraulic pump and valving, you could be in for some serious surprises,

Good luck with the oil change
One more thing others use 15w40 diesel oil in lieu of GL-1 but in England they can buy non detergent multi weight oil, I never found it over
here,
However at this stage in the life of these old tractors any new oil is good,