Pontiac Solstice Performance book says Mobil 1 75-90 synthetic for both the trans and diff.
GM (Service Info) says part number 89021806 75-90 dino oil. (But synthetic for the diff.)
The little I've researched on this subject leads to Amsoil Severe Gear as being pretty good, but a lot of that opinion is based on this... http://www.amsoil.com/lit/g2457.pdf
which no doubt is a little biased but I'm not sure they can cheat a whole lot on these tests.
Then there's some info saying don't use synthetic in this trans, not sure how serious of a problem it would be, I doubt it could hurt anything but then again these trannies are pretty bullet proof using the stock dino fluid. I've seen lots of problems using synthetic oils in dirtbikes (combined engine, clutch and trans oil) the clutches really don't like synthetic oil. Not sure what these syncros are made of, if they're metallic it shouldn't be a problem, if they're any kind of fiber synthetic oil could be a problem. (???, I'm NOT a trans tech so I'm not presuming to be any kind of expert on this subject.)
Here's some more for you... (from GM Techlink)
Using the incorrect type of transmission fluid in Aisin manual transmissions may affect the sealing ability of the seals. An incorrect type of sealer may not be compatible with the transmission fluid or may not have the correct characteristics for sealing the affected components.
TIP: Refer to SI documentation for correct sealers, adhesives, lubricants and fill quantity for each transmission you service.
Aisin manual transmissions require the use of p/n 89021806 - Fluid, m/trns 75W-90 GL-3, 1QT (89021807, 1L, in Canada). The use of the correct fluid is required to assure the desired shift characteristics and compatibility with adhesive sealers.
RTV is the only sealer that is compatible with this fluid. The use of anaerobic sealers on Aisin manual transmissions is not recommended.
Mine's only got 26k on it, but I was still planning on changing it soon. I might just use the stock recommended fluid since there doesn't seem to be any problems that better or different oil would need to fix. The other factor is if I need to warrantee the trans or diff, having a receipt showing I serviced it with the recommended GM fluids would be better than having a GM rep seeing purple fluid in a broken trans or rear end. If I were to go non-oem oil I'd probably trust the Pontiac book saying Mobil 1 synthetic 75-90, second choice would be Amsoil Severe Gear. All 3 of those oils have a lot of reputation behind them that I would certainly trust. (Never been a big fan of Amsoil though.)
edit... found more info saying stay away from GL-5 gear oils for trans use, evidently it's easy on gears but hard on syncros. Amsoil synthetic trans fluid is GL-4 and looks like a better alternative for the trans than the Severe Gear oil. Something like the Severe Gear or GL-5 oils would be fine for the diff. I'm staying with GL-4 fluids for this trans. Mobil 1 Synthetic 75-90 is GL-5, maybe the Pontiac Performance book says it's ok for the trans since they really don't care about long term durability for a car that they're setting up for racing only.
I guess my conclusion is OEM or Amsoil Synthetic 75-90 trans/gear oil for the trans and Severe Gear for the diff. Doesn't look like it's worth going non-OEM for the diff since GM recommends their good synthetic gear oil anyway, but it might be worth trying the Amsoil Synthetic (or any other synthetic that doesn't say it's GL-5) for the trans.
BTW, Royal Purple Max is quite a bit higher viscosity than most of the others, even seems higher than what data sheets are saying 75-90 is supposed to be. Could be a problem shifting when cold. The Amsoil trans oil is quite a bit thinner but still right in there with what 75-90 is supposed to be. I didn't find a spec for the OEM dino 75-90, it would be nice to know how thick it actually is compared to these others. 75-90 isn't 75-90 in all brands, that's for sure. Royal Purple Syncromax is WAY thinner than what is recommended for this trans.