We traditionally call "sludge" is a substance caused by motor oil exposed to
normal motor temperatures plus water and other combustion contamination that
forms a mixture of various compounds that participate out of the oil. Over
time these compounds can coke-up and form a solid mass.

Synthetic oil tends to avoid this by doing a better job of keeping the
compounds in suspension and a better job of dealing with the acids that form
when the compounds are exposed to water. Any detergent oil will quickly
disolve the hardest deposits if change frequently enough, perhaps every 500
miles, till the sludge and hard mass is desolved.

More likely a problem caused by 1. poor design of the lub system, 2. changed
fuel mixtures, 3. failure to use an oil that can deal with the changed fuel
mixtures, and 4. not letting the motor come up to full running temperature.

The reports you hear about spark, coolant, etc., are pure magic.

Richard.