The reason for ATF is that the WC T5 runs roller bearings under the gears, which makes less friction, which means a lighter oil...ie ATF. The WC T5 also runs carbon fiber blocker rings, which is what helps it achieve its higher HP and TQ ratings over the earlier T5's, which ran gear lube due the the lack of the above upgrades.
If a transmission's tolerances, seals, bearings, etc was designed and tested around a specified fluid, ATF in this case, your only going to shorten duty life, hinder performance and end up with a leaking and/or broken piece of crap transmission if you run gear oil...
My question is this... Some of you idiots on here have no clue and apparently no common sense regarding simple maintenance, yet you are the first ones to post some stupid crap like you know what your talking about. Why would you tell someone to do something that is or may be detrimintal to their vehicle and/or themselves?
When in doubt refer to your owner's manual. You do still have your owner's manual don't you. I think a lot of owner's fail to realize that after you get past the section on 'how to fasten your seatbelt' the owner's manual has a wealth of information like a list of all the lubricants and their specifications and capacitites. Also contained in the manual is a list of bulbs, fuses, fusebox layout, and some useful general info.
Very wise advice from a wise man here. In more than one instance (I'm kind of anal about fluids), I've seen a parts store manual quote a different type of fluid for a component than what the owners manual quoted. Ditto for capacities.