Gear box tool - what size and type

Hello all
after Reading most of the threads here about gear box issues, I think I need to top up my gear box oil.
Yes I do have a leak and yes I have noise. However on looking at the books and finding the filler plug, what type and size do I need? I have tried a torx T50, spline mm but they don't fix correctly. The best fit was a 8 mm hex but still feels lose.
Thanks

if i recall correctly its a torx 55 mate

It's a Torx 55. You'll also need some method of getting the new oil in - I use a 500ml syringe.

Thanks for the guidance chaps

Ive just changed the gear box oil on my 300tdi auto and refilled it down the dip stick tube