ISX engine 800k miles burning oil

[SBTA]

Hey one of my isx engine burns around (1-1,5)gallons of oil after 8000 miles from oil change on light loads...and can up to 3 gallons with heavy loads in mounts...have been to a couple mechanics to find out oats going on...i have done an oil analyze a the Speedco they find light amount of coolant in oil, cooler charge have beed replaced, they have done a pressure test in coolant system they say its normal...i have few leeks from deep stick, from the compressor tail but i don't believe can dump from that point in so big amounts...so i explain how is it with my words...for any ideas beside for an overhaul, my mechanic say if it burns more than 2 gallons that internal parts...i got this truck with 540k and it they have done some repairs inside of engine... (previously owners )
THX