Ive just put a 5speed (w4 i think) G/B into my hilux but the problem is that it leaks oil out the tailshaft end.
I did top up the gearbox oil once I put it in but not enough to make it spew back out the filling hole.
Also it played up for a while the other day and it was really hard to get outa 3rd like the clutch wasnt workin proply but it only seems to happen occaionsly when its hot ?
Sounds like the rear seal is in need of replacing. You should top up the oil til it starts to come out the filler hole, that way you know it has reached its appropriate level of oil.
If the oil is low then you may have problems with shifting and the like and it may only be highlighted in third gear because it has been worn a tad more that the other gears/syncros.
Fill it up and replace the rear seal where the tailshaft goes in or the yolk comes out.
Replacing the seal at the tailshaft end will most likely fix the leak. (also if its a 5 speed it will be a w50 or something, not a w40, w40's are 4 speed )
Also when I go about filling up manual gearboxes I take the shifter out, and remove the filler bolt on the box, and then just fill it up slowly until the oil comes out the gearbox filler hole on the side of the box (seem the easiest way to do it if you ask me!).