you need to check exhaust back pressure to determine if the cat is clogged. There is an adapter fitting that screws in the o2 sensor threads and a gauge that indicates the pressure as the engine is accelerated.
the following should have been replaced already, fuel filter, Maf. All the lines flushed, injector cleaned. Good luck
James 1:25 Ordered from Faulkner Chevy #NKCF02 1100 5/8/9 2000, 3300 7/16/9 TPW 7/20,GOT MY (low) VIN! 4000 7/24/9 ,Picked up 8/7/9!!! Signed Litho #45. Car signed by FBodfather 2011. Service: Jennings Chevy in PA. We don't "drift", kiddies... we POWER SLIDE! MOTM/COTM Sept 2012 Pro Patria Vigilans. Click the pic.. I don't tolerate fools.
If was just diesel then i wound say buy it...but since its drinking oil like a two stroke you should probably just stay away Unless you can get the can for about $500 and you are prepared to dump a couple thousand into it.
Diesel will get past the rings easily and quickly. This will dilute/mix with the engine oil , washing the oil off the walls, bearings etc very quickly damaging bearings. I wouldn't trust that engine any longer than it would take me to sneeze! I wouldn't even buy it at 72k without the diesel adventure.....
Can't really add more that the others above have mentioned other than diesel generally wont hurt a thing and just drain tank and change fuel filter....do the reverse (gas in diesel engine and catastrophic!). I would assume there are deeper issues...but if the price is right, a good used engine is affordable to drop in....and nothing in the PCV system would have been effected...there is no PCV valve on these...just a PCV fixed orifice barb in the rear of the passenger side valve cover.