Just though I'd post up my impressions of the ear oil cocktail that Andrewtech Automotive uses for their Subaru 5-speed gearboxes. Andrewtech is the most respected Subaru transmission builder in the nation, so I trust their recommendation.
The cocktail is 3 quarts of Redline Lightweight Shockproof, and 1 quart of Motul Gear 300 oil.
Before putting in the cocktail, I'd get awful grinding in 2nd, and 3rd gears. The gearbox felt sloppy and gooey when cold, and was overall a pain to shift. Now, I have yet to get any gear to grind, and the gear engagement is sooo much more smooth and easy. It was pricey for all the fluid (~$100), but totally, 100% worth it.
I should add that I have the stock 160k transmission, and whatever gear oil was in it before was cheap generic gear oil.