Just a few items to add to the mix:

There are different threads used on sump ports on various
airplanes. Make sure you order the right one. You will find 1/8" and
1/4" NPT as well as 7/16NF, and there could be others. The NPTs uses a
sealant like fuellube, and the machine thread styles use an O-ring.

You CAN buy the tiny replacement O-rings for the valve end. We do.

The dark flecks are often bits of bugs. If you leave the cap off
the tank, or the vent is big enough for some insects to enter, they
will. They like gasoline fumes. I have taken numerous bugs out of
tanks and drain valves. Kepp those caps on whenever not dipping or
refuelling.

Fuel is filtered, usually, three times between the tank and carb. A
finger screen in the tank (coarse), a finer one in the gascolator, and
another at the carb inlet fitting. Teflon tape shouldn't be a problem,
but I still don't like it. FuelLube is better.

Dan