This foamy stuff in your oil is the result of too many short trips.  
If you don't get the oil up to operating temperatures it can't burn 
out all of the moisture and you get this foamy stuff.  Every time the 
engine warms and cools moisture condenses in there.  One good highway 
run should fix it.  If I HAVE to make a few short runs in a row I make
sure to do this before I store it for the night.  I find a bike is a 
good long term cure for this problem. :)
-- 
James A. Brill Jr.         \\  //
jbrill@unlinfo.unl.edu   \\ \\// //      If you're not outraged
University of Nebraska    \\//\\//     you're not paying attention.
free-lance homo sapien     \/  \/