The book of MF says, it is okay to use either the Permatran, or gear lube, in the front axle. Even if they did not suggest it, I would still use the gear lube.
You want to use a gear lube with the "EP" designation. These gears can generate very high pressures that really need this kind of protection.
You can switch to synthetic at the 50 hour service. Recommending using mineral oil for a "break in" period is becoming a thing of the past. Today's mineral oils are extremely good lubricants, other than they break down faster, and don't have the temperature latitudes of synthetics, there is little difference.