O.K.
Cosworth T5: Dexron II
Cosworth MT75: MT75 fluid
RS2000 MTX75 up to 1995: MT75 fluid
RS2000 MTX75 1996 model up: SS75W/90
Note: I recommend the use of a full synthetic 75W/90 on "big tooth" motorsport gearsets, but on all of these boxes you will need to get the box warm before expecting a quick gearchange