^^^ Jimabeana is correct...
Gear oil that is syntetic will definately say so. The factory wants you to run a 90 weight gear oil of a specific specification in the axle. Most oils that you get will meet or exceed that spec, especially synthetics.
The multi-range oils (technically multi-viscosity) will have certain chemical properties built into their formulation that allow them to act like the lighter weigh oil when cold and the higher weight oil when hot.
As an example, people used to switch up the type of oil that they ran in their engines in hot weather. If the manufacturer called for SAE 30 or 10W30 for general use, some would go up to SAE 40 or 10W40 for hotter weather and SAE 20 or 5W20 for colder weather as the lighter weights are "thinner" (not really, but that is about the best descriptor of how they act) and flow out easier when cold, and the heavier weight oils will remain "thicker" or retain their consistency better when hot.
In minus 20F temps, a 90W gear oil is about the consistency of Karo syrup - and it doesn't do much to lubricate your gears until it warms up enough to flow. It can actually almost stall out the vehicle on a cold start becasue it simply does not flow - so most folks in cold weather climates use a 75W90, which will flow in cold weather like a 75 weight oil, but lubricate under stress or heat like the 90W. Same goes for gear oils that are 75W140 or some such formulation - they can take hotter running than the 90W oils.
With synthetics, though the numbers are simialr (75W140) they will actually flow BETTER than 75 in cold weather (it takes something like -80 to even start to thicken up the syntetics) and they will also take the heat better that a 140 will, but becasue they are so "free" in their pourability, they also lessen friction internal to the gearset - improving economy, etc.
So, find some oil that meets or exceeds the manufactures level - preferably synthetic (try another parts store) and you'll be happy and still be in specs for oil.
