Thanks for the replies.  I should have mentioned this is an 82 aircooled
Vanagon, so that knocks out the explanations relating to water and the
coolant system.

The "short trips" scenario doesn't apply, since the van has lately been
used only on weekends for trips of 1 hour or more, and then sits all week.

Question is, is the brown gunk a problem or anything to be concerned over?