Google is your friend...
Straight off Garrett's website,
"An oil restrictor is recommended for optimal performance with ball bearing turbochargers. Oil pressure of 40 – 45 psi at maximum engine speed is recommended to prevent damage to the turbocharger’s internals. In order to achieve this pressure, a restrictor with a 0.040” orifice will normally suffice, but you should always verify the oil pressure entering the turbo after the restrictor to ensure the components function properly. Recommended oil feed is -3AN or -4AN line or hose/tubing with a similar ID."
