Factory stock engine blocks were designed to be mass-produced
economically and to operate in a passenger car environment. Original
equipment oiling systems are typically restrictive and feed oil to the
top of the engine first. The common practice has been to use a
high-volume oil pump to offset these shortcomings and help the main
bearings get more oil to survive.

Dart aftermarket blocks have been designed with a less restrictive oil
system which feeds oil to the main bearings first; a “priority main”
oiling system. Many of us in this industry are inclined to use a
high-volume oil pump due to history. However, this causes excessive oil
to the top of the engine. Restrictors are then used in the lifter
galleys to reduce the amount of oil to the top.

Modern lifter designs typically have small oiling orifices which
restrict the oil reaching the top of the engine. Using a
standard-volume oil pump with current lifter designs and the Dart
priority-main oiling system eliminates the need for restrictors,
simplifying engine assembly and taking the guess work out of oiling
system design.

Using a standard oil pump and today’s lifters, without restrictors, makes
engine assembly easier and eliminates the guess work of determining
restrictor size.

From Dart Machinery