Not sure exactly were to steer you about the source of the moisture. I can tell you that with a 4x4 it is common to get water in the differential during a water crossing because the hot housing and cover are shocked by the cold of the water, contract, and allow water to come in when they contract.

Keep in mind that the main difference between a gear oil and a motor oil is the additive pack. API viscosity is different for a gear oil and a motor oil thus a 76w-90 gear oil will be similar in viscosity to a 10w-40 motor oil. For example, the Viscosity Index of AMSOIL's SYN3 equivalent (MCV 20w-50) is 161 while their Severe Gear 75w-90 is 165.

 

The main benefits of using gear lube is the additional additive package which helps to reduce shearing/scuffing.

As far as the 'viscosity' of gear lube, many people fail to realize that a 75W90 gear lube has about the same 'thickness' as a 20W50 motor oil. The below chart will give you a better idea when dealing with motor oil and gear lube;



Also, I wouldn't use a motor oil in the manual transmissions of my cars or trucks, and I won't use it in the manual transmission of my bikes either.

There are different API rating for gear lube, ranging from lower protection GL-1 to GL-5 which is rated for 'severe conditions'.. Any GL-5 rated gear lube would work in the manual transmissions on Harley's, but with the Sportsters you need to be a little more selective as the same fluid is used for the transmission and the clutch pack. Because of that you want to use a fluid with a low enough moly content that it won't cause an issues with the clutch. Too high of a moly count might cause issues with the clutch plates.

A good synthetic GL-5 rated gear lube is Mobil 1 75W90 gear lube, and it is used by many that own Sportsters. UOA's show that the moly content is only around 60 ppm, which is less than many 'motorcycle specific' motor oils.

Here is a little more information dealing with gear lube API ratings;

API GL-1, oils for light conditions. They consist of base oils without additives. Sometimes they contain small amounts of antioxidizing additives, corrosion inhibitors, depresants and antifoam additives. API GL-1 oils are designed for spiral-bevel, worm gears and manual transmissions without synchronizers in trucks and farming machines.

API GL-2, oils for moderate conditions. They contain antiwear additives and are designed for worm gears. Recommended for proper lubrication of tractor and farming machine transmissions.

API GL-3, oils for moderate conditions. Contain up to 2.7% antiwear additives. Designed for lubricating bevel and other gears of truck transmissions. Not recommended for hypoid gears.

API GL-4, oils for various conditions - light to heavy. They contain up to 4.0% effective antiscuffing additives. Designed for bevel and hypoid gears which have small displacement of axes, the gearboxes of trucks, and axle units. Recommended for non-synchronized gearboxes of US trucks, tractors and buses and for main and other gears of all vehicles. These oils are basic for synchronized gearboxes, especially in Europe.

API GL-5, oils for severe conditions. They contain up to 6.5% effective antiscuffing additives. The general application of oils in this class are for hypoid gears having significant displacement of axes. They are recommended as universal oils to all other units of mechanical transmission (except gearboxes). Oils in this class, which have special approval of vehicle manufacturers, can be used in synchronized manual gearboxes only. API GL-5 oils can be used in limited slip differentials if they correspond to the requirements of specification MIL-L-2105D or ZF TE-ML-05.