OE lifters are a performance replacement for stock hydraulic roller lifters. Preload is meant to be setup similar to stock. Like stock, they are a ball valve style lifter.

OE-R lifters are specifically designed to perform at higher engine speeds. When engines are equipped with a hydraulic camshaft, high rpm is limited by the improper position of the internal piston as the lifter inevitably "pumps up". This improper location results in open valves and therefore leads to lost power. These lifters are almost required for any 3800 car shifting above 6000rpm because they help with the typical 5600rpm dip. OE-R lifters are preloaded and have to be installed between 3/8 and 3/4 turn (vs. 1/2 to 1.5 turns on stock lifters). These are a shim valved lifter like the ZZP and LS7 lifters. They have less range adjustment so preload is more difficult to setup. It's recommended to use a pushrod length checker.