Most conventional transfer cases take and require the use of SAE 80W or SAE 80W-90 GL-5 gear lubricant.
Some other full time transfer case systems use SAE 10W-30 or 10W-40 engine oil and some transfer cases on vehicles use the same transmission fluid your transmission takes.
If we knew what vehicle you have or what transfer case you're asking about the transfer case fluid then we could tell you more exactly what your transfer case takes.
If you have an owners manual to your vehicle it should say exactly what fluid your transfer fluid case takes.
But usually it's either regular gear oil or even regular engine oil.
My truck takes regular gear oil for the transfer case but another vehicle I had said to use transmission fluid.
So it can vary from vehicle to vehicle.