Dont put oil in the green plughole it is not the gearbox filler & leads directly to the clutch!
the gearbox & transfer box fillers ars both arrowed in the photo, the early gearboxes are filled through the speedo cable hole
its part number is 020 301 129D – you will also need part number 020 301 132 – they should cost about a fiver at the VW Dealers
btw – Part number 02A 301 115A is for an 02C (cablechange) gearbox & will not fit the rod change box.