The oil might have 'thickened' with age, or if it was low.
Any idea how much oil you added?
As you say, getting it warm will ease the shifting problem.
Spray some penetrating oil such as WD40 around the drain plug threads and let it soak.
Repeat until you are certain that it has 'penetrated'.
Get everything warm and try to undo the drain plug - don't force it.
If it won't budge get a hot air gun and heat the area around the drain plug and try again.
If it really won't budge after the WD40 and getting it hot, then use a flexible tube through the filler/level plug to 'suck' out the old oil - use a long length of clear tubing so that you can see when the oil is coming - if you get it right it will syphon itself once you get it over the 'hump' - especially if the oil is warm.
Measure how much comes out (if you can) to get some idea how much was in there.
I believe the capacity is 6½ pints. You might not be able to get it all out but diluting it with fresh oil will help.
I do not know what oil you need - probably 80 or 90 grade and probably not EP (which tends to be for rear axles).
EP additives which contain phosphorous/sulphurous compounds are corrosive to yellow metals such as the copper and/or brass used in bushings and synchronizers.
ST90 would be my guess.
Edited to add:- you might have a plate on the side of the gearbox (rectangular with six bolts) which is where a PTO would be fitted.
Removing that would release the oil . . .
The gasket for the plate might be cork or a compressed 'paper' - though the shape is simple (and standard) and you could use dense card - anything that doesn't allow oil to seep through (and which isn't affected by oil). A motor factor (parts supplier) should be able to supply suitable material if you can't find something.
I see they want silly money for genuine gaskets! :-
http://www.ebay.co.uk/itm/PTO-GASKET...p2054897.l4275
Cheaper in the States - but how much postage?
http://www.ebay.co.uk/itm/M35A2-PTO-...item1c3ca28b86