Oil restrictor fittings may be necessary on Ball-Bearing style turbos, and are not always required. Check the oiling system carefully and make sure there is not another problem (such as inadequate PVC venting or oil drain restrictions) that is causing the oil leaks and smoking, before restricting the inlet side of the oiling system. If restrictors are used to reduce oil leaks and smoking, start w/ a large restrictor and work your way down carefully to eliminate damage to the turbo (from over restricting the oil).