Truck Repair: Cummins N14 Using Oil, excessive blowby, cummins n14

Question
My driver reports that his truck has used 4 gallons of oil in the past 3 days.  He does not see any "major" oil leaks, just a small one around the pan gasket, that we were aware of.  Previously, it would use a gallon of oil per 3000 miles.  Now it's using 4x that much.  All the temps and pressures are in normal range and truck seems to be pulling strong as always.
Any ideas where the oil may be going?

Answer
I need to have answer to the following questions.
1. What is the present speedo reading on truck and at what reading did you last carry out a major overhaul?
2. What type of major overhaul was carried out the last time?
3. Does engine emit blue smoke?

High engine oil ccnsumption can occur as a result of:
1. Worn out piston ring/liner
2. Leakages through the valve oil seals
3. Worn out turbocharger bearings/shaft
5. Excessive blowby.

Please check the above, I am sure you will find solution.

Regards,