"Jay Honeck" wrote in message news:OaATc.292632$JR4.125853@attbi_s54...

I can understand condensation, but sludge?

With regular flying, and scheduled oil and filter changes, what is the
source of sludge in an M20? Combustion bi-products?

Well they say that cars driven a short distance suffer from sludge
buildup
when the acidic moisture, created as a combustion by-product, remains
in the oil. I believe the same is true in an aircraft engine. Any
place
where entrapped moisture routinely remains in contact with oil will
form
sludge. Look at the vent tube on an airplane with considerable time
on
the engine. Some have sludge build up because they cool the crankcase
vented gases low enough for the moisture to condense out and remain
on the inside walls along with a film of expelled engine oil.
Depending on
where an air/oil seperator is located it too can run cold enough to
become a collector of moisture and therefore suffer from sludge
buildup over time.

My M-20 is located high enough and out of the engine cooling airstream
to
get and stay relatively hot. Since I can't visually inspect my M-20 I
take the proactive measure of cleaning it out each year.
In fact if my memory serves me correctly I believe the instructions
for
continued airworthiness on the M-20 STC says to clean it periodically.

GaryP