Up to you, but ZF who made the box reckoned 80 weight gear oil was what was needed, presumably so any light shavings fell to the bottom.
If you lie under the front the van, you can easily feel - and see - the filler plug. Pop it off and top up with a squeezy bottle full of gear oil.
Mine is a February 73 undid the two screws lifted up but pushed the plastic collar down ...the plate then lifted up and gave access to the bung . The plastic collar was a bitch but it helped having it with a slot in ...don't know if it was a mod by the P.O . It went back into place with a bit of fettling and the two mushroom head bolts and captive's were greased up as good as a porn star .
I try'd to do that , but after having a bit of a mini go at it ..i gave up as i could only think i was going to be covered in gear oil .
I had a good feel about from the unders and got a good feel of the plug but I confess it all seemed a bit impossible it’s location in on the forward side of the steering column shaft and is not really visible unless I’m not finding the right contorted position… i couldn’t even get a screwdriver in to prise off the bung… then i thought i wouldn’t get any oil in or get the bung back … so went to the inside cover approach … I will give it another look at the weekend.
The O ring for the Red plug is a 15x2 mm one, oil is used instead of grease because any swarf turns the Grease into a grinding paste
Bet your horn stops working because you knocked the wire off where it goes onto the outer column with all that fiddling ... been there done it.. You do NOT need a puller for the arm on the steering box, despite appearances it comes out in one piece. You only need a ball joint splitter for the drag link. And again, I would have pushed filth into the steering box if I had not removed and degreased it beforehand. This was on its way to a seal replacement so I had to remove the arm to get at one seal.
I like that gizmo ...i have a 200ml syringe with long tube attached... i will mod it to hold the tube at the right angle of the dangle like you have.