I make my own oil fow restrictirs out of either # 6 or #10 Brass set screws and then open them up to the size I want. You have to drill and tap the oil passages for them so I do, the rocker arm oil passages can be done fairly easy, same thing on if you want to put the restrictor in the oil passage in the heads instead of in the block I had one old SS 440 motor that the Rod Shop had built originally, they had used the brass set srews in the oil passages between the main bearing bosses and the cam journal bosses with around a .030 hole in them, I wouldn't do that on the #4 cam journal oil passage though if I had modded the oil passage to the heads to restrict oil to the heads BTW, I do groove the #4 cam journal on the cam to oil the rockers full time on all my roller cam motors to help oil the valve springs full time That ends up making the differences with the rocker arm type on the size of the restrictors, bush or extruded aluminum rocker arms don't need as small of a restrictors as the full needle bearing roller rocker arms do Same thing on the valve springs, high pressure solid roller cam springs need a lot more oil on them than a non solid roller valve spring will need at the same RPMs