I bet there's many old service stations that have bought bulk gear oil in 55 gallon or larger drums and it takes them decades to use it all up,if they aren't a high volume repair facility...
Only thing I'd be concerned about is condensation,but if its still sealed ,that shouldn't be an issue...I once asked an Amalie factory rep if oil can go "stale" or has a shelf life..he couldn't say for sure,but did remark some of the cases of older straight weight oils they dont sell much of lately,dated back to the '80s,this was in 1992...and he added "I dont doubt there is some really old oil still buried in our warehouses across the country--some of the boxes are probably worth more than the oil,to a collector"...he also added that if anything,the additives would settle out of the oil maybe,but the oil should still be OK..
I used 10 quarts of Sears "Spectrum" 10W-40 I found in my basement my dad must have bought in the late 70's in my K5 years ago,it was probably 20 years old--looked nice and golden with no water or separation,so in it went!...never noticed any harm...