I am one of the members of the "other forum" rich posted these pics on, and am quite strongly against such positioning of a turbo oil return.
My original reply:
I'm questioning the position not because of crankcase pressure, but because of oil levels and pressure differentials. For fluid flow to occur, there must be a pressure differential, and the fluid will flow from the higher pressure area to the lower pressure area. Having such a low oil return port will reduce the size of the pressure differential between the oil return hose and the sump, and so slow the flow of the oil. If the flowrate in the oil return hose is low enough for the smaller pressure differential to have a small enough effect for the turbo to maintain a high enough oil flow rate, then all will be fine. I've never measured the rate of flow in the hose though, so couldn't guarantee that. So if it were mine, I'd be playing it safe and raising the oil return. If Paul's happy with the way he's done it, and has experience with similar engines, I'm sure all will be just fine.
Why do people think that placing the return above the oil level would create a problem with crank case pressure? The pressure in the crank case acts on the oil surface, and so with the oil return below the oil pressure, you're giving it the crank case pressure AND the pressure head from the oil depth to work against when trying to return the turbo oil to the sump. Poor positioning in my opinion, and it seems quite a few people on here agree. A breather is fine returning below the oil level as it doesn't deal with continuous flow (provided the engine isn't seriously sick!), but that bears no relation to a turbo oil return.