These fittings allow you to effectively limit oil flow to your turbo, thereby minimizing the potential for damaging the oil seals in your turbo from over pressurization. These oil restrictors are available in 0.020", 0.030'' or 0.040'' for your ball bearing turbos, or as a straight non-restricted fitting for journal bearing turbos. Garrett GT ball bearing turbos like lower oil pressure and sometimes the built in restrictor is not enough. With these -04 AN oil feed line fittings you can ensure your GT Ball Bearing Turbo is getting the correct oil pressure. Recommendations: 0.020" - Xona Rotor Turbochargers & FP ZEPHYR, FP ZERO, FP Ball Bearing Green/Red/Black 0.030'' - Garrett Ball Bearing GT28/GT30/GT35 0.040'' - Garrett Ball Bearing GT40/42+ Thread Size: 4AN to 7/16 x24 (male to male)