Hi Shaun
This is what I think you want:-
https://www.opieoils.co.uk/p-6363-c...synthetic-car-gearbox-oil-formerly-taf-x.aspx
Castrol Syntrans *
Transaxle 75w/90 synthetic
The "which oil" thread has been done to death over on the Lotus Esprit forums, and the oils originally spec'd by Renault and Lotus have disappeared from sale (Mobil SHC 630M and Castrol TAF-X). It's worth bearing in mind:-
Oils spec'd by Renault / Lotus were based on what was available at the time, and this was before the general availability of synthetic oils.
Oil standards have been changed, and some additives are banned owing to being subsequently identified as hazardous to health.
The replacement oils, had they been available at the time to Renault / Lotus, would in all likelihood have been approved, but as they had yet to be developed they never made it onto the "approved list"!
The UN1 is a transaxle, not just a gearbox. The oil needs to handle the heavy load between crownwheel & pinion, as well as the movement of gears, bearings & synchros, and shift rods. The need for a heavier oil to protect CWP can mean a less-than ideal shift quality.
*Note: there is also a Castrol Syntrans
Multi-vehicle 75w/90 which is Not the Same Thing.