[v8volvo]

All good points... I am a little concerned however by what sounds like a sudden change in how the engine is running and the apparently rapid onset of the oil pressure symptoms.

Faulty oil pressure switch is fairly common in these (switch will leak internally and cause misbehavior) but if you are in fact hearing noise from the valvetrain, that would suggest the oil pressure loss is real, and serious. Though agreed of course beneficial to verify with a mechanical gauge.

In addition to what has already been well covered I think you could give a little more attention to the turbo which you mentioned early on. That is one of the few things that I could imagine causing sudden increase in oil use and smoke coupled with a loss of oil pressure, if the turbo bearing and/or seals were failing. I think it is an outside chance since by now a more catastrophic turbo failure would probably have shown itself if there is indeed an issue there, but no reason to dismiss it until you are sure. Easy check would be remove the inlet pipe from the turbo cold side and see how much axial and radial play in the shaft, any signs of mechanical damage, etc. More effort but more information would be to disconnect the exhaust downpipe from the turbo hot side outlet and look for evidence of wet oil, etc.

Otherwise it is hard to think of much that would cause sudden development of your symptoms (at least not in an otherwise well-maintained engine that has not recently been run dry of oil or overheated or suffered some other disaster). Things like worn main and rod bearings happen very gradually over time and anyway are usually very robust in the D24 even to extreme high mileages, again short of some outside factor causing failure. Oil pump wear is something we have seen cases of, but usually that is observed as difficulty building oil pressure after a cold start. Hot idle oil pressure problems are less familiar, though of course possible. Again though, it is just hard to see what would take an engine with 320k miles that may have some wear but is basically in stable condition, in consistent use, and being obviously carefully looked after by you; and suddenly put it on a fast decline.

Thus why a turbo failure could make a little more sense.... Otherwise, maybe some debris passed thru the oil pump and damaged it suddenly? Or oil pump inlet tube loosened up or cracked and it's sucking some air resulting in reduced pressure? But none of those would explain the increased consumption, unless multiple problems were worsening at the same time simply due to age and general wear. I suppose that is not out of the question in an engine that has lived a long life and had unknown care prior to your ownership.

One final idea would be to take a sample of crankcase oil (assuming it has not been too recently changed to show anything) and send it in for laboratory analysis. Not sure if those are readily available in your area but there are several that offer the service by mail here in the US and it is not expensive. If something in the engine is failing or abnormally wearing, the analysis may be able to highlight it. Different types of excessive metal present in the oil can point to specific problem areas, e.g. chrome for piston rings, aluminum for pistons, iron for cylinder walls, tin or lead (iirc?) for bearings, brass (??) for turbo shaft bushing...... excess silicon for a failed air filter letting in dust and tearing up the internals.... excess glycol % pointing to failed oil cooler or headgasket.... etc. Kind of like a blood draw for the engine.