Just passing on a few things I learned while working in a major's research lab in my youth with some updates I have learned from the lab guys over the years.
Assuming one is using a decent detergent motor oil, the number one cause of sluge is water. This is typically associated with a motor used in cold weather without getting the oil hot enough to burn off the water. The water can form acids resulting in direct attacks on rings and bearings and can speed the formation of sludge.
One wants the oil to neturalize the acid and hold all the solids in suspension. This is not an easy task, but modern oils can do a very good job, especially some of the better pure systhetic oils such as Mobil 1.
If I drive in cold wet weather and have lots of short trips and lots of idling, I would want to change that oil more frequently, even a good synthetic. If I took mostly longer trips, and knew that my oil got hot enough to burn off the water, extended oil changes, up to 15,000 miles or more with Synthetic oil, can be most reasonable.
Those motors that are most prone to sludge formation are typically motors that have poor designs, usually in the head areas, that run too cool, thus promote sludge formation even though the block is getting up to running temperature. Also motor designs that don't promote proper venting of the water vapor can be a motor with sludge issues.
In this case Synthetic oil and reasonable oil change schedules are called for. Toyota seems to have gone this way.
Typically, a good detergent oil should remove sludge over time if changed frequently enough, although I have resorted to a specific solvent detergent additive to remove solid black formations in the head area of a Ford 6 that never had the oil changed in 100,000 miles, but don't recommend that for a motor you have not abussed to the point of needing a rebuild anyhow.
These have been presented to me as general statements based on extensive lab tests, but I don't mean to suggest that this is all there is to it.
Richard.