special oil from AM Gen in gear hubs

[RubberDuck]

That special oil that you speak of is not for the geared hubs but for the Differentials with factory equipped Eaton Lockers. It's a synthetic gear lube with a friction modifier additive. It's not exactly $40 per qt. but more like $24.55 per quart but at $10 per qt. it's still a good deal if you have lockers be it Aulburn or Eaton.
FWIW, I would not reccommend running Mobil 1 gear lube in your Geared Hubs lest you want leaking hubs in your future. Don't get me wrong, I am a Mobil 1 fan myself but the Mobil 1 Gear Lube is quite a bit thinner and the hubs will almost always leak after time. Lucas Hub Oil is the only way to go IMO. It has a seal conditioner additive that will virtually eliminate Geared Hub leaks.
Again, IMO/experience.......