50 to 1 may be a little too little oil. Aftermarket ABOS manual calls for
1/2 pint (8 ounces) per gallon (128 ounces) 8/128= 1/16 for your Chrysler.
That may or may not be correct but I would not run the engine on 50 to 1
without verifying with manufacturer's instructions.

The leakage may be residue of exhaust. What color/smell is gear lube...same
as leakage or different?

Using up the fuel that came with the motor in the tank may seem like a good
idea but that may lead to a gummed up fuel system (if it is not already)
Gasoline ages rapidly.It does not improve with age. Best not to buy more
fuel than you will use while it is still fresh. It is also a good idea to
measure fuel and oil so that each batch of gasoline/oil mixture is the same
ratio for consistent engine performance. Gasoline that is 60 days old ain't
what it used to be.
Using a preservative in last fuel used before storage may be a good idea
to prevent crud and corrosion. Probably an excellent idea preservative is
as good as claims.
There are products intended to "fog" engine with preservative oil before
storage, and also products to "free up" rings. Both products intended to
overcome problems of short seasonal use and long off season storage.

Not knowing service history is a very good reason to check water pump
impeller. Chrysler took over an existing outboard motor maker (West Bend)
in 1965 with an established and proven product line.
Louis