OK, I'm not certain, but this is what I guess it is for based on the design. There is a oil pressure valve at the oil filters that will partially bypass some oil towards the oil cooler when oil pressure exceeds 45PSI. The oil going towards the oil cooler actually goes back towards the oil pan. Without a restrictor, the oil cooler circuit represents a very low pressure circuit and too much oil can get bypassed through this loop (ie. oil pan->pump->cooler->oil pan) and starve the engine of oil. So I think you should keep the restrictor, unless you are going with a different oil cooler circuit design (an alternative is to go from oil pan->pump->cooler->engine->oil pan, which is how the Stillen cooler for the NA works).
