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That's the reason they were building up sludge is neglegent owners not changing the oil often enough.
Check under the oil cap on top of the motor for sludge buildup and on the bottom of the cap, youll see it on there.. Ours has a little but were keeping track of the oil changes and it's doing good so far.
Also pull the dipstick to check the color of the oil, if it's black dont touch it, browns ok but black oil with no oil change sticker on the window is a good sign it hasnt been taken good care of.
That doesnt mean it will be a bad car and if its not a car with all maintenance records from someone you trust just get it checked out, have the compression tested on all cylinders and pull codes if the check engine light is on to be sure its not something costly.....
If you get the car cheap enough and its nice enough its worth buying even if it has a sludge motor. Just take care of it and change the oil often and replace the motor if it ever pops, and if it doesnt congratulations.