Some years ago, Hitachi dry film did a study of the "scum" left behind in the
developer, and found that it was a product of two things, dry film residues
that have reacted with water hardness, and defoamer.  The level of scum
varied a lot with both different dry films and different defoamers, and in
some cases, resembled an oil.

Many defoamers contain either silicones, or kerosene, or even, heaven forbid,
some phosphate esters.  These last are famous for eating developer parts.
 Any of the afore mentioned defoamer ingredients can give an oil.  There are
defoamers that give, honest to god, no scum or oil at all, with any dry film.
 The disadvantage is that they tend to be expensive per square foot of film
processed.

Perhaps the best compromise is to use a pure polyglycol defoamer.  It gives
low, but not zero, scum and/or oil,  but has the limitation that it works
over a smaller temperature range than a defoamer with some of the serious oil
producers in them.  This means that if you allow the defoamer sump to cool
off, it may foam until it reaches  operating temperature again.

Do I have to tell you that my honorable employer, RD Chemical Co.  is proud
to offer various flavours of defoamers, and is happy to discuss the virtues
and limitations of each of these types of chemistry. 

Thank you for asking.

Rudy Sedlak
RD Chemical Co.  415-962-8004