Dartâ„¢ Priority Main Oiling System

In a priority main oiling system, the oil from the engines primary oil galley is supplied first to the main bearings and then to the cam, lifters and top end. This ensures that the critical crankshaft main bearings receive adequate lubrication for highly stressed racing applications.

All Dart blocks employ priority main oiling systems.

Factory OE blocks, in contrast, typically feed oil to the top end first as it is easier to machine them in this fashion and factory blocks are not designed for the demands of racing.

From  http://www.dartheads.com/encyclopedia/priority-main-oiling/