This is off the net....shows oil is a fault for causing sludge.
Most common cause of sludge problem
As complex as the issue of sludge formation is, the most common cause of the sludge problem is the oil itself.
More exactly it is the use of either substandard oil like API SA or just using even the top quality oil for just too long.
As "obvious" as this may be, it is not surprising that general motoring public is not aware of either of these two problems.
Even if your "Owner's Manual" recommends top API or ILSAC quality motor oil, there is not any assurance that the local cut rate "quick oil change" outlet actually will use it in your vehicle, more often than not they will use deeply discounted "surplus" bulk oil whose quality is either substandard or as bad as API SA !
The second most common reason is just running way too long on the motor oil for the given type of service.
Another not too often disclosed fact is that during the ILSAC or API engine tests on motor oils, the oil life is measured in HOURS, and yet only very few vehicles sold anywhere in the world have ENGINE HOUR RUN meters.
The normal motor oil recommended service frequency is specified in miles or kilometers, but the motor oil life is in hours of service.