If you do a search, you'll find multiple threads on GL-5, GL-5/MT-1, and GL-1 oils...
Spicer actually recomends 50wt for the transmission. Your TM says 90wt for diffs, transmission, transfer, winch as it works in all and allows for single sourcing.
GL-1 meets the specifications for the units.
GL-5 is designed for hypoid gears with greater pressures, but works just fine (cost more per gallon though) as long as you get a version with the MT-1 certification. The only way to be totally sure if it does meet MT-1 is either to see the cert on the container, or have the tech info from the manufacturer (and make sure that it is for that exact product/lot as things change...and technicalities count).
In my area, Agri-Supply is the least expensive, Tractor Supply comes in close. Walmart/Sam's Club isn't bad when you catch it on sale. Napa/Carquest/O'Rielly's are higher with NAPa being the most expensive. Best deal of all is to buy in bulk directly from a distributor...
Heavier weight oils can be used, but you increase internal wear and reduce economy due to the extra force needed to rotate parts in a more viscous environment. Multi-weights are fine as well, but are more expensive than the single weights (at least in my area where we have a lot of folks running agg equipment and/or older construction/stationairy, etc. that uses older single weight lubricants and, thus, ensure a ready supply on the shelves).
