A few things come to mind that would cause the oily residue. The person filling the tank was not very neat, and there was fuel spilled around the filler cap that drained down to the bottom of the tank. There could be a leaking cap gasket. Then there could be a leak in the fuel supply system from the threaded hole in the bottom of the tank to the carburetor. If it was leaking at the tank connection, then that fuel can end up in the tank mounting ring.
These motors have a primer instead of a choke. The primer sprays a small amount of fuel mix out of the rotary valve port which ends up on the inside of the lower cowl. Not the most environmentally sensitive design, but bear in mind this design was from the late 1930’s.
Here is a very helpful site for “T” series motors: Fuel Tank and Fuel Tank Valve Cl (pochefamily.org)
Improvise-Adapt-Overcome
