90W gear oil works fine, and so should pretty much any multi-vis gear oil (85W-90, 90W-140, etc.). I use 85W-90. There's nothing in the idler or steering box that's adversely affected by the EP compounds. Some people use Penrite steering box oil, and some swear by STP.
There was a recent thread on replacing the seals in situ. It can be done (the idler's easy enough to remove, but not the steering box). Some of the replacement seals are thinner than the original; people seem to do OK stacking two of them.