I've read several posts of guys having luck with Amsoil 75-90 gear/trans fluid.
One guy experiences -10F and says no problem.
http://www.amsoil.com/shop/by-produ...-transmission-and-transaxle-gear-lube-75w-90/
Standard Transmission recommends Amsoil. My G56 is on its way back from them and that is what they are sending with it.
DClark