All looks a bit fake to me considering it's supposed to be Italian oil and you can't easily get it in the UK anymore! Link to Ebay item - OLIO CAMBIO ZC90 BRITISH OIL.

Back to the topic, gearbox thoroughly drained and Millers GL3 put in this evening. After 5-6 miles, the box really does feel different, there is no need to press a little against the syncro now, each gear just drops in nicely. The Millers GL3 90 grade is definitely thicker than the 80w90 it replaced (which did smell sulphurous). It is also thicker than 20w50 that goes into the TR5 so I wouldn't be sure about using that as a replacement (as the Haynes manual suggests). Worth seeking out the correct oil, many thanks all!