I would not put that oil drain back on the car. Oil will back up big time. One thing people need to know, the oil that comes out of the turbo is not a nice flow like a water faucet. It is an aerated mess. This is why the factory drain tube has convolutes, to help the oil collect and drain back in to the valley. I have yet to see an aftermarket SS braided drain that 100% eliminates the kink. Also, you'll note, the stock drain tube doesn't have a direct 90* at the block. But rather a smooth transition out of the turbo and into the block. (see image below) This is critical. The ID of the drain is also critical. Journal bearing turbos can typically take 80psi before showing signs of leakage aka before oil gets by the oil ring on the exhaust side. DBB turbos with the internal restrictor such as the air cooled DBB chra from PTE can take more, but we always recommended keeping oil pressure below 80psi to be on the safe side. Bison is correct, there are no 100% sealed oil seals in either style turbo, but the dbb is a little bit more resilient due to the contained cartridge, vs a jb chra.
As far as controlling the amount of oil to the turbo, I would not put a restrictor on either style turbo. Doing so will shorten the life of the chra.
If this is a jb turbo, there are two things that can prolong the life. (if you have filtered oil feeding the turbo already)
1. Slowly lift off the throttle when getting off the gas from WOT, vs slamming the throttle shut.
2. Have the turbo retrofitted with a Ported Shroud S style compressor cover, to help keep it from surging under half throttle and most importantly, to help keep the rotating assembly from spinning 110k rpm one direction, then immediately reversing and spinning 110k RPM the other direction when you get off the gas. Or, install a blow off valve, but that is not typical. Without those two, on a jb turbo, you will eventually "knock the thrust" out of the jb bearing system. When this happens, the oil seal, really an oil control ring (piston ring as mentioned earlier) get's taken out and the thrust bearing is next to go along with the actual bushings. When you have signs of oil in the compressor cover, or the compressor wheel rubbing the compressor cover, the thrust is gone and it's toast. Running a turbo with the thrust knocked out or compressor wheel rubbing the cover is only blowing metal into the engine and contaminating the oil with further metal particles.
I highly recommend, always running a ported shroud S compressor cover on any journal bearing turbo going on a street driven Turbo Buick. I also highly recommend the factory style oil drain tube, if cracked, replace it with an OEM tube. And having filtered oil with ZDDPlus to the turbo at all times.
-Patrick-