E30 Zone

My 325i touring has a noisey diff, whining over 40mph, it's not too bad at the moment, was going to check the oil level. Anyone know the allen key size I need to remove the filler plug, looks like a beast.
Secondly any preference on diff oil? and is there any extra lubricant I can add to the oil to get a bit more life out of the diff, extra lubrication?
Also, how do I know if the diff is still working, the reason I ask this is because my touring is very tail happy in the wet, much more so than the 2 325i saloons i've had, is this just a touring problem or is my diff no longer doing it's 'limited slip' bit?
Any opinions
Cheers
K

right its a 10mm allen key i think. give the bung a good wallop before you try to undo it.
oil for the lsd.. a lot of different advice here, so i went and paid a fortune from a main stealer for the genuine oil onlt to see the can and see that it is just synthetic gear oil that i was gettin x85w90.
if its tail happy i'd be thinkin your diff is workin, all that extra weight of the tourin is prob helping ya get the arse out! enjoy it til ya hit the end of the rack!!