"Oil sludge" refers to a thick and sticky substance that forms when oil breaks down or gets contaminated. It can clog up engines and pipes, causing damage and reducing efficiency.
Full definition
My 1998 Honda Civic LX appears to have
oil sludge in radiator overflow tank.
What's the preferred, non - chemical / non-additive method for
removing oil sludge from an engine that's still installed in the vehicle?
I'm not sure a compression test will be an accurate representation
of oil sludge.
[8][9] In February 2009, five separate class action lawsuits related to the
alleged oil sludge defect were consolidated to the District of New Jersey.
It even takes off
bath oil sludge, or the cream oil bodywash sludge, eeuuw.
If you want to figure out whether it really is a blown head gasket or not, here's a Scotty Kilmer video demonstrating a test kit, but if there's
oil sludge in your coolant, you definitely have some problem.
Otherwise the water (and whatever else) that has collected with the oil in't removed and can lead to
oil sludge.
The oil sludge I'm sure would make it very hard for the engine to pump oil through the narrow passages.