Engine sludge

[chinoy]

What is the cause of engine sludge? Recently when I was travelling to abu dhabi, my car made a noise and broke down. I towed it to the dealer. They kept my car for 3 days and called me to report that it happened because of sludge in the engine. They told me that the oil change in my car was not enough. How is it even possible? I get the oil change every interval as per instructed by them. Is there any other reason for engine sludge?