I was told by a rebuilder that owns WC engineering in the USA to use Redline 75w90 GL-5 part#58304 (no friction modifiers). I changed it in my un1 a couple of weeks ago and is definitely an improvement from what was in there but have no idea what was in the unit originally.
I would take Mikes advice and see what others are using, maybe use my info as a reference.
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.
I run Millers CRX 75w90 NT in my UN5's, seem to give decent protection without making the shift too bad (Syntrans makes the shift better when cold but resulted in more wear on the diff when I tried it, but bear in mind the '5 is a much bigger diff and under way more loading than road/track cars will see)
It has served me well in my R21 with the Chris Cole 1 piece shaft upgrade. It also has a Quaife TBD in it. Over 20K miles with about half of that on track. I recommend it for a R21 without a clutch type LSD or for a TBD type limited slip. If you have a clutch type LSD then you can add friction modifier to it. Friction Modifier is also available from Redline as well as info for its use.