MTF 94 for the 75/ZT box, and 10/40 as an emergency top up.
Dexron III is specified for the LT77 gearbox as used in Sherpa vans, and with modified versions fitted to Land Rovers without bothering to think about the later R380, which is also a MTF94 user like the PG1 gearbox.
The long and short is simple, use MTF 94 in your Rover 75/ZT manual gearbox without additives and it will work perfectly
Brian
