Ok someone can prolly answer this one easy. I hit 62k recently so I have been doing the maintenance and such. I put redline oils in the the tranny and rear end. My clutch started slipping about 10 days after and its seems really weird b/c when im WOT its almost like there isnt a problem at all. But if Im just normal driving ; I hit boost and my clutch slips.. I have a new clutch to put in but I want to understand this problem before I just ignore it and move on.
From what I have read on here and nasioc STi's don't really like anything other than the subaru tranny fluid. I think Will pointed it out in another thread sometime also.
Tranny fluid cannot affect the slipping of your clutch (unless it is leaking from the tranny and somehow getting onto the pressure plate). I think the trans oil would affect shifting more. If your clutch is slipping and you have a new one, replace it.
you could have hot-spotted your flywheel... thus requiring you to get it resurfaced before the new clutch is installed, this causes the flywheel to become "glazed" and causes it to slip. You would be able to tell by color distorsion on the contact surface. Before I replaced my clutch, it was slipping but not at WOT... so who knows.