This has been discussed several times on this Forum.
The Operators Manual for the Case 800 and early 830, both with Case-O-Matic, says to use 10W Non-detergent motor oil for the Trans/Differential.
We recently rebuilt the brakes on our 830, which included new seals on the brake shafts and a flush-out of the Trans/Diff housing with fresh 14 gallon of oil. The answers I received, for the same question, was to use the newer oil formulations... such as Case HyTran.
I did just that and the Ol' girl shifts better than ever. No grinding when going into gear from neutral and it's much quieter, no bull-gear whine. HyTran has my vote, although our CIH/CNH dealer gets nearly $60 for a 5 gallon pain... Wow!