is your WARN engine light coming on? a blown turbo very well may not turn on the light,
but why i ask about an engine miss is if its running perfectly smooth but still blowing unburnt oil all over the back of the bus I would more suspect a turbo like you and cowlitz suggest.. if it has a miss in it and is only hitting on 6 or 7 cylinders I would suspect an injector O-ring or possibly over-filled engine with oil getting by a valve guide seal, or possibly the intake side of the turbo blowing oil into the air intake which would make incredible amounts of smoke..
however i wouldnt expect to see unburnt oil if it is going into the intake unless its so much that cylinder is not firing....
the low oil pressure is likely caused by one of a couple things.
1. a blown turbo will opbviously be leaking oil out to the point the pump cannot keep up
2. the oil level has dropped very quickly due to oil being burned and blown all over.
3. a bad injector oil seal will cause the computer to ramp up the HPOP to try and recover from the leak and will make your lube oil pressure (dash gauge pressure) go lower due to an increased volume required.
4. oil pressure drops as engines warm up.. esp if they are worn.. if the bearings are bad the oil P will drop.. however at thios point id suspect injector seal or turbo as the reason for oil pressure loss..
-Christopher