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Cat C15 MXS with low oil pressure and oil consumption.

Hello, I have a C15 MXS engine in a Kenworth dump truck that has a low oil pressure issue. The operator said that the oil pressure guage was reading 10 psi. He didn't tell me about the oil consumption until I had left the unit for the day. I checked the oil level and it was a little low. It was recently serviced as well. I topped up the oil and ran the engine. The engine was cold and was at 30 psi on startup. As the engine warmed up the oil pressure slowly dropped to about 15 psi before I shut it off. I hooked up a manual pressure guage and confirmed that the dash was reading the same as the guage. There are no external leaks on the engine. The operator also complained of excessive blowby from the road draft tube. I removed the valve covers and ran the engine. All looked good up top. I drained the oil and removed the oil pan, removed the pick up tubes and pump. I dis-assembled the pump and there is a bit of wear in the pump but not so much as it should be down to 10psi. I checked all the piston cooling nozzles and all looked good. I rotated the engine and couldn't see any signs of broken rings in the cylinders and all had good crosshatch. I dropped one rod bearing cap and one main bearing cap. I also removed the upper bearing shells and inspected. The bearings have normal wear and no signs of starvation or heat. I checked the bypass valves in the oil filter head in case they were stuck or plugged but they were clear and cleaned. Originally I was looking for low oil pressure issues and not oil consumption. The operator told me that it used 5 liters of oil in a day ( 8 hours). I was going to check the IVA oil check valve but now I'm concerned with the oil consumption as well as low oil pressure. I did check the flash codes and the only code was a low coolant code. I checked the coolant and it was just a little below the float. There is no oil contamination and everything I can see looks good. I would probably go after a broken ring or excessive cylinder wear for both low oil pressure and oil consumption but I can't see any problems in the cylinders. The engine has almost 13000 hours on it and I am not sure when the last bearing roll was done but it has had one at least. Sorry for the long story but I wanted to include everything I could. If you have any ideas on this problem, please help. My next step will probably be looking into the turbocharger shaft seals as both problems could stem from there. No complaints of boost loss and that is the first thing the drivers notice. Any ideas? Thanks