As you probably found, there are strong supporters of using each. While I only used the Castrol option in my own over the 4 years I had it, I would have been comfortable with Redline MT90 given the years of trouble free usage described by those folks.
Either oil is good. It also seems to depend on your local climate and how hot or cold it is. I am using the MT-90 here in the Northeast and it seems fine. The biggest complaint will be how it feels when shifting but that can also be because your linkage is worn and/or needs some adjustment. Always remove the level plug FIRST! If you take the drain plug out first and can't get the level plug out you can't put the oil in and now the trans is empty. Sometimes those plugs are really tight and hard to remove especially if you don't have the right tool.
I've also used both, in 4 pots i found that they both work well and i also use MT90 in the Elan. I did however go with Syntrans on the V8 due to it being the Lotus recommended oil (purchased through JAE) and as noted i found the shifting much smoother with the castrol product on the V8.
I currently use the MT90 in my V8, but wouldn't mind trying the Syntrans if I thought it might smooth out the shifting a little.
Can I add the Syntrans straight after draining the MT90?
What does the Syntrans fluid make the shifting smoother?
As @carbuff mentioned “if you can find it,” I don’t think either is exactly what I used. They seem to have changed the packaging or branding since I bought it last, and neither one looks obviously right to me. The Castrol stuff evolved at least slightly over the past decade or two. I would not be surprised if one of these is appropriate, but I’d prefer to avoid guessing.
I only used Castrol Syntrans Transaxle 75W90 and I change it every year, no matter what milage or whatever.
Lotus described that Taf-x was superceeded by the above newer oil. It is not 100% entirely the same spec, but Lotus did test it as far as I read, and they said good for it. As far as I know, Lotus did not try the MT90 oil. I don't know anyone here in Denmark who use it, so cannot speak of it.
I called Castrol Denmark, and they couldn't answer on any of the questions regarding these oils. So I also called Castrol UK, and had a lenghy talk with them and while they admit that there were a little bit different additives and a little bit different values of certain chemical substances in TAF-x compared to Syntrans Transaxle, they also said it's the closest to Taf-x, that they have on sale today. Taf-x bottles are now scarce and seriously expensive, so I choose not to track it down, and instead use Syntrans Transaxle fresh from the producer, and change it every year, regardless.
The Castrol Syntrans Transaxle 75W90 is not expensive in Denmark. I usually buy 10 1L bottles so I change on both cars, and have extra if I go far away. I keep it in little Castrol soft cloth 1 Liter bags, that have velcro on the back, so it sticks to the luggage compartment vertical wall in the Esprit. Same goes for engine oil. In that case I only use Castrol Supercar, which is the newest formulation of the old Castrol RS oil, with many itterations in between, incl. the introduction of titanium additive. It hs a surface strengh near double of any other oil on the market. So I choose htat, in case I should loose oil, oil pressure or similar and because I only start and use the car once in a while, and finally it's the exact oil that Lotus recommends for the Esprit. I called the factory some years ago and asked, as well as asking Castrol. I change it every 5000 km's or every year, what ever comes first. Always.
Beware when ordering, as Castrol makes many near similar oils regarding the gear box oil.
Regarding the engine oil, I have only used this for many years, also on my other cars, just in other vscoisty, and I have never had any mayo in the cylinderhead, cam towers, bad smell, nor any oil problems, bad pressure, no detectable wear, near no oil use at all, ever. just my two p.