If it's the NP205, that case was spec'd from ATF to 80W90, so mixing the two oils you have would work.
Personally, unless it's chain driven, I like to use gear oil. I use Supertech 80W90 in the Dana300 in one of my project vehicles.
I would be concerned with the ability of the thinner oils molecules to keep the moving parts apart with the high loads inside a transfer case. If it were mine I would not chance it. Oil is cheap, transfer cases are not.
Jim, are you referring to the protective film that EP gear lubes typically provide? If so, I agree.
I'm not sure gear driven T-cases operate without boundary lubrication (as opposed to any type of hydrodynamic action taking place - maybe under low loads?).
The spec back in the day was probably SAE30. Delvac 15W40 would be a good substitute and flow better when cold.
The 203 doesn't have a pump like other chain drive cases, so I wouldn't go too thick. Your Frankenbrew would be fine.