The most common cause of oil leakage on a perfectly good turbo (called a "forced leak") is due to excessive crankcase pressure. Since your oil drain tube carries returned oil into the crankcase of the engine, it relies on gravity to carry that oil from the bottom of the oil drain port seamlessly into the engine crankcase. If for whatever reason, you are building an excessive amount of crankcase pressure and are trapping that pressure in the crankcase with inadequate ventilation, the "pressure" will send itself up the oil drain tube into the bottom of turbo. Since there is no gravity left to carry the incoming oil out of the turbo bearing housing, the oil is forced to leak into the housing and it then gets burned in exhaust and the result is smoke as you have experienced.

In most cases, people put on restrictors to "mask" the real problem.
Less oil goes in, less leak or chance of leak. But also less oil into the bearings risking possible starvation.

To begin to properly troubleshoot the oil pressure vs oil drain issue, you have to measure the oil pressure during turbo/engine operation, but the measurement point is inside the restrictor (between the oil restrictor and the turbo bearing system). This will give you the net actual pressure taking into account pressure drop caused by the current restrictor. See if that oil pressure curve across the engine rpm is inline with what that turbo needs (per turbo manufacture) for properly lubrication.

Otherwise, we don't recommend blindingly adding restrictors of various sizes. It's all a shot in the dark.