The Stovebolt Site Forums The Shop Area Driveline Leaking 49 Chevy truck

"Is this transmission oil? Where would I add this oil? Is there a seal that can be replaced? How much of a problem is this? "

"Is this transmission oil?" - most likely (unless the rear torque-tube seal is bad)

"Is there a seal that can be replaced?" - the smallest ring-seal (rubber) in these sets, at least (the other seals/gaskets in the set might also be leaking).
https://www.classicparts.com/1935-54-U-Joint-Ball-Gasket-Seals/productinfo/92-111/
https://www.chevsofthe40s.com/detail/6244/Chevrolet_Transmission_Gasket_Set_3Speed.html

The Shop Manual describes the procedure (in Transmission Installation, steps 5. a-e).
You will have to open the u-joint cover, and disconnect the u-joint

if needed: u-joint assembly and lock plates/bolts Chevs of 40s
Classic Parts U-Joint
Classic Parts lock clip (you need two)
Classic Parts bolts - you need four