Re: Oil return line...
The side that goes to the turbo is a standard NPT. That one is either a 1/4 NPT or a 1/8 NPT. The side that goes to the steel braided hose is either a -3AN or a -4AN.
Zach, none of those fittings are compression. One side is NPT and one side is Flare (the nipple looking side).