ISX engine 800k miles burning oil

[Waterloo]

Fix the leaks and go from there... The 1 - 1.5 gallons sounds normal for an older high mileage ISX between oil changes. The 3 gallons if pushed hard, I could see it. They do not run forever. And then throw in your oil leaks... I am guessing one at the compressor? You can lose a bit of oil between oil changes there if left to leak. Have you changed out the crankcase breather filter? You said you had coolant in the oil? I wonder what your pressures are like internally? That is just off of the top of my head from what you wrote, smarter individuals will hopefully chime in.