As previous posters have said any Dexron 11 or 111 ATF is OK - if you are changing the ATF there are several things to be aware of like how to get most of the oil out and how to fill the box - draining at the plug will only let about 4 of the nine odd litres out and some boxes fitted to the petrol model have no filler tube, just a level plug. Some advocate a disconnect of a cooler pipe and running the engine to pump out all the fluid and some like me just drain what you can but do it again after a week.
ITOCUK tech forum can advise on this and many things. Good luck and by the way you have a legendary tow car!