Hi, All Responders:
THANK YOU ALL FOR YOUR REPLIES!
I've learned so much from you.

Herewith are the results.
(My original request is at the bottom for reference.)

1) Stuck rear wheel drum.
After I backed the shoes off to the maximum, the drum
just slid off the axle. (The linings were still OK to pass
the inspection.) The linings had eaten into the drum, and
a ridge formed at the outer edge. There was also quite a
bit of encrusted rust. This ridge was preventing the drum
to come off. I filed the ridge off, and now everything is
honky-dory.

2) Mayonaise-like sludge on the oil filler cap.
Most likely, this is the result of condensation water
since lately I have been driving mainly 5-6 mile commuter
trips. Later, I'll make the "lick test" to verify that
the sludge does not result from a coolant leakage.
(Coolant makes it taste sweet. This was suggested by one
of you.)

Thanks again to all of you. Alex

==========================================
CC: My original message:
Please help!
What to do? I cannot get off the rear wheel drum
on my trusty '78 Honda Civic (just 100,305 M on it)
to check the brake shoes. The nut is off, and I've
loosened the brake adjustment, but the drum does
not come off. (It can be rotated.) I've tried
prying with a screw driver between the drum edge
and the back plate, but no luck.
The other rear wheel drum just slides off.

The other question I have is about the oil filler cap.
Especially when it is cold outside, the oil filler cap
is covered with some Mayonaise-like substance.
What is this emulsion? Does it do any harm?
I would appreciate any suggestions. Especially,
about the drum.
Thanks, Alex