From the '91 service manual:

I'm quite confident it was the MT90. I spoke with a few others around the globe that had the same issue (one in UK & US). It is not immediately apparent. For the first year or two, shifting felt fine. The synchros simply wore prematurely. It came to the point that I had to resort to ATF to prolong rebuilding the box for a year or two.
 
It may very well be something else at work, but given that fresh rebuilt transmission lived & died on MT90, I'm not down for another round or willing to recommend. If I were to buy another MR2 & it had MT90 in it, I'd change it immediately.

Only gl4 caltex oil at super cheap is vmx-m which is a 75w-85. No blurb on the back and overlaid sticker on front. Will check repco.

Cheers

Agree, sometimes it's best to take what the American's on the MR2OC have to say with a pinch of salt.
By the way your build is absolutely amazing. That larger supercharger is going to be incredible. 

Went to Repco and Auto1, both only had the Castrol GL4 VMX-M 75W85, and 2 Penrite GL4 olis, 75W80 and a 80w85.
 
Caltex welshpool have the GL4 75w90 ($40.15 for 4litres) they said its unlikely any shops would stock it as its not a popular oil.
 
Cheers

Well I just filled up the transaxle with MT90.
I literally filled it up because it started streaming out again.
Anyway, it's full so I'll see how it goes.

You get the Caltex oil from selected petrol stations. There's one near me. They even sell drums....lol

Thanks for the heads up.  I did ring another large caltex but they didnt stock it.  Might try some others closer by.
 
Cheers

They list on their website who stocks it.

Another reccomendation is the Penrite Gearbox Oil as mentioned via Road and Track. I've given this a go and find it ok. Just make sure you get the right stuff, shame it isn't the cheapest oil to come by! Previously I had the Castrol Fully Synthetic ATF which seemed fine also.