XH-21 transaxle oil formula?

Getting to the point of reassembling the 220. Service manual specifies Castrol XH-21, but this no longer exists. Castrol rep was unable to give me specs as its been out of production over 20 years. Don Law has specs, but he had his own “better” oil made by a British company, and he’s not talking. I suspect he is at the point of requiring all 220s to be shipped to the UK for oil changes.

The major problem is the transaxle and hypoid gears. The hypoid gears need a GL-5 EP lubricant, and there are many to choose from. But since the transmission shares the oil, GL-5 is not great for synchros, The EP additives (sulfur, phosphorus, zinc) tend to eat away at any yellow metals, like the bronze in the synchros. There are many differing opinions on the web, and they all seem to make sense even if they are diametrically opposed.

Royal Purple makes a GL-5 hypoid gear oil that it states is totally safe for synchronizers. So does Amsoil. But I don’t want to be their guinea pig.

What are XJ220 owners using for transaxle oil if you are not getting it from Don Law?
Is the correct oil 75-90 (I suspect it is, as 140 would be very thick for the transmission gears)?
Motor oil is much easier, but is there a preference for a specific brand?