Originally Posted By: ChemicallyChargd
The cold temp spec is the same on both, and the amsoil high pressure lube hangs onto the ring gear teeth very very well as the pinion teeth sweeps over them, as others said, if you aren't either key word 'frequently taking track days, pulling around car loads of people ALL the time, then I wouldn't want the thicker stuff, it's just more drag to overcome, and less torque to the wheels. Heavier gear oils are really made for heavy duty trucks where the pinion gear will exert very large amounts of torque on the ring gear under towing conditions, where the lighter oil just can't "hang on" to the teeth as the pinion sweeps over them, absolutely not a concern in a light vehicle.
-srv
Correct info above on the use of heavier differential fluid for more towing and higher torque situations. Also, gear oils are designed to work chemically on the iron to prevent scuffing, pitting, and galling, even when there is no lubricant film present. The GL-5 gear oils produce a chemical layer of iron sulfide on gears, which is sheared off when there is metal-metal contact. This prevents straight iron-iron contact that produces scuffs, cold welding, and other very very bad things.