Don't use normal ATF, the nearest fluid made these days is a atf, but a specific one, it is CASTROL TQ-D, halfords sell this.
Interestingly a different oil is specified for WC and NWC boxes, NWC being a 70 weight gear oil, while the WC uses a ATF, was dexron 2, but no longer made, mercon was ok to use, but again no more, castro, tq-d is the closest, using the wrong stuff will screw up the blocking rings.
tabetha