If I had to guess, I'd say either would be fine. Universal provides adequate lubrication in high horsepower tractors, and 80w-90 provides good lubrication in non-pressure lubricated units. I tend to go with the book, as there is usually a reason they recommend a specific oil.

One thing I think everybody should try one time is to check a gearbox that uses 80w-90 once it is good and warmed up. You'll be surprised how much it expands, and how thin it gets once it is up to operating temperature.