There is a HUGE thread on NASIOC about USC (Uncle Scotty's Cocktail). It was like a 1000 pages and basically a bunch of people did oil analysis of it and found after 10-15K miles (as stated above) it becomes acidic and corrosive. DO NOT USE IT! I thought this was common knowledge, as USC was proven to be bad a long time ago.
Any of these would be fine:
1) Motul Gear 300
2) Subaru Extra S
3) Andrewtech Cocktail (3 qts Redline Lightweight Shockproof, 1 qt Motul Gear 300)
Personally, I just use straight Motul Gear 300 and have been using it for a while running 332whp/338trq on my stock 5-speed tranny. I just don't like the idea of mixing oils (chemistry experiment) as each oil has its own properties and not everything mixes well. But, Andrewtech is one of the premier Subaru Transmission builders/repairs in the USA and I trust their judgement. Turn in Concepts/TIC swears by straight Motul Gear 300. Many swear by straight Subaru Extra S, it is a nonsynthetic.