ATF is common in manual transmissions, the T5 and T5Z in Commodores and falcons use it, the T56 6 speed uses it along with countless FWD boxes. Castrol transmax Z is what Malwood auto have been recommending to be used in all of there supplied T5 T56 etc. I have used the transmax Z in a number of RWD manual boxes including cars I have previously owned with good results. Although it's typically the over fill that cures the ****ty shift in the T56.
A GL4 75w85 or 75W80 will also be fine in an MR2 (is what I use in my track car)
Avoid GL5, as it's does not supersede GL-4, it is different spec and will cause shifting problems and can damage the syncro's.
The simplest thing to do is not ask your local pimple head at the parts store.
Just pick your favorite brand and go to there lube guide. Ignore the Toyota manual on this, as it was written a long time ago and oil tech changes.
http://www.datateck.com.au/lube/fuchs_au/http://www.datateck.com.au/lube/castr_au/http://www.datateck.com.au/lube/valv_aums/http://www.datateck.com.au/lube/NulonAus/