Tust me on this, don't use Castrol synthetic 75/90, you will get the famous 4th gear grind. I've used this before, along with several other members, and most of us started having this problem. Once I switched back to Subaru Extra-S, the problem was fixed. Gear box is smooth as butter. I bought mine off of ebay, and some of the vendors on here sells it as well.
No need for a flush, just drain and fill. Same with the rear diff.