This may start another p!ssin' contest, so this is my opinion based on experience and study. What you want is straight, non-hypoid mineral oil of the proper viscosity. Tractor Supply carries it under the brand name Traveller Ford Tractor Transmission Oil. NAPA also has suitable oil under their own brand name. Stay away from anything intended for both transmissions and axles and transaxles as it will have hypoid additives that, over time, will damage your br*** synchronizers. Modern oils are for modern transmissions that don't have br*** parts. If you want to re-seal your transmission check with Mac Van Pelt at http://www.vanpeltsales.com/. Mac is the obsolete Ford transmission guru and a great guy to deal with. He'll have the seals, etc. you'll need. Good luck!
According to this guy; Understanding Borg-Warner's R-10/R-11 Overdrive Transmissions - Engine Builder Magazine Lubrication Caution: Do not use anything higher than GL-1 in the overdrive transmissions. Modern gear oils are up to GL-5 or higher now, but the modern detergents in the gear oil will attack the bronze parts inside of the transmission. And DO NOT use modern synthetic gear oil in the overdrive transmission. It is too slippery and the overdrive sun gear will not engage. You will have to dis***emble the transmission and clean every part to get all of the slippery film off all of the internal parts. Not a fun job!
I have been using the NAPA brand for several years with no issues. https://www.napaonline.com/en/p/NHF65201
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