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cheers, i actually found a parts place that knew what i need, amazing, plus the packaging told them it was for VW gearboxes, but thank you for replying anyway as this has just confirmed it for me. i did "phone a friend" while is was at the parts place as the drive didnt look big enough, as he had a allen key that we knew fitted, and asked him to measure it, oo err, and he said it was 17mm as well. so im all set to do the gearbox oil change now. im converting the hypoid oil to fully synthetic, which im probably going to do by dropping the hypoid totally running the fully synthetic for a couple of thousand miles and then changing it again.