Do NOT use the universal tractor fluid in an allison. The c-spec is ancient. Totally inappropriate for any tranny today. The only reason TSC carries that stuff is because there are a lot of old tractors out there that can run on it. These tractors are low speed engines with high clearance pumps. They use the same sump for both transmission and hydraulic feeds. You want a Dexron3 or better, minimum, for a Allison 545/643, and ideally a TES-295. Note that the fluid says it is "recommended" but does not say it actually meets any spec. In other words, it has no official approval for that use.
The 15w40 motor oil is ok. Not great, but if you change it frequently it will do alright. But for about 3 bucks a gallon more you can get Delvac or Delo on sale at WallyMart.
"You can finally say you have enough horsepower when you leave two black streaks from corner to corner"
(Mark Donohue, famed TransAm driver)