"transmission fluid" = "gear oil"
Cobb Tuning talks about ShockProof a bit in this article:
Redline is sorta funny about the viscosity on it, as it has a much higher film strength compared to flow compared to other oils - that is, when you talk about the protective barrier between parts it works like a thicker oil but when you compare it on the basis of how well it flows from place to place it is more like a thinner oil. Hence this odd description from redline:
(from http://www.redlineoil.com/redlineoil/spgoti.htm)The Heavy can be rated as a 75W250 Gear Oil, but has the lower internal fluid friction of an SAE 75W90. The LightWeight can be rated as a 75W140 Gear Oil, but has the lower internal fluid friction of an SAE 30 motor oil. The SuperLight can be rated as a 70W90 Gear Oil, but has the lower internal fluid friction of an ATF. These ShockProof lubricants provide much greater viscosity in the thin layers between the gear teeth, but have very low internal fluid friction, which means very little power loss for the protection achieved.
So Cobb recomends the superlight but you could go clear up to the heavy and still get enough flow. I have the SuperLight in mine, but we put the LightWeight in my buddy's Impreza 2.2L w/ excellent results as well.
Basically it's a matter of how much protection you need - the SuperLight gives you better protection than standard oils anyway, so (at least in Cobb's opinion) you're better off using it and reaping the efficiency benefits.