Not sure exactly what specific 80W-90 oil you are referencing, but many of the gear oils on the shelves of auto parts stores (at least the ones by me) contain an API service rating of either GL-4, or GL-5.
While the EP (Extreme Pressure) additives contained in GL-4/5 rated gear oils are great for reducing wear in hypoid gear sets, most are not particularly friendly to any yellow metal components in the transmission - i.e. synchronizer blocker rings....
The following link has a lot of interesting information on this topic:Gear Oil
While there are exceptions, many of the data sheets for these oils contain similar warnings.
At one point, I might have leaned toward finding one of those "exceptions", but after looking at some of the spec. sheets for some of the plain old 90 weight GL-1 oils, it looks like the manufacturers are putting in a pretty substantial additive package (corrosion inhibitors, anti foaming, etc.).
That said, here is the data sheet for the "Tractor Supply Traveler Ford Tractor 90 weight oil". Ford Tractor Oil
While the top of the data sheet says "API Service GL-1" (which is good for you) , further down the page it says: "Performance Standards: GL-4" - which is confusing to me. If it meets GL-4 performance standards, does it include any EP additives?
Perhaps you could shoot them an email telling them your application and see what they say. The fact that many on this site are already using it without any issues is obviously good sign, but it would not hurt to double check.