Hah! I didn't even know 70W-90 existed.
From
here:
Vis @ 100°C, cSt 15.6
Vis @ 40°C, cSt 90
EDIT:
Eh, after looking up the 75W90 on AMSOIL's site again, it also says 87 cST @ 40C.
Here is the datasheet. The 129.7 cST came from their "long life synthetic gear lube" product, not their normal 75W90 synthetic.
That, and I can't seem to find a good source that actually sells 70W90 with a quick google search...