yes this oil will work, old mercedes take regular cheapo industrial SAE90 gear oil. btw - the gland nut is always that tight, did you use a proper socket on it? do you have access to an impact gun?
to loosen the fill plug - you can heat up the diff case a bit around it, then touch some beeswax to the threads and smack the filler plug with a tool inserted in it, the jarring will break the bond and the wax will lube them, I use a 24 inch breaker bar in a swift action to break the plugs loose - if the plugs are rusty, first clean them up nicely and then put some valve cut paste on the tool so it bites harder in the plug.