Depends on what color the gunk is.
If it's black, it's from carbon. Usually blowing by worn or broken rings.
If it's creamy, white to tan, it's from water. This can be either from an internal coolant leak from a cracked head, headgasket or intake (on a V8). Or it can be water from condensation, where the operator hasn't changed the oil as often as necessary for the way the vehicle is driven.
The claims about certain oils causing sludge is one of the biggest myths in the industry. There are no unbiased, scientific studys that have proven that brand A causes sludge. It all goes back to engine maintenance. Either the oil hasn't been changed often enough for the driving conditions or the engine is in need of mechanical work.
Whatever the case, it is best not to attempt a chemical removal of the sludge. Leaks can occur, as can bearing damage when the dirt goes through them. If you wish, mechanical removal is best. Pull the valve cover(s) and pan and attack with a scraper. Removal all the loose gunk before putting things back together. And don't worry about the power loss claims. They aren't enough to measure the difference in terms of milage.
