Getting the same amount of volume (6 qt that you put in) when doing oil change does NOT always mean your engine is not losing oil or there is no problem. I do lab test on used engine oil for my customers and I see any where from 5% to 15% fuel contamination on used engines. Fleet truck operation companies do this all the time as well and that's how they determine when they need to do the oil change, not mileage alone. Even if you get 6 qts out, you may have two problems (at least):
1. Oil consumption
2. Fuel contamination
The oil analysis will also show you the % or ppm level of various kinds of metal particles which tells you which component of your egine is wearing out such as rod bearing, piston rings, cylinder liner, etc. and it also shows coolant contamination as well. My cost is about $10 per test.
For hihg mileage trucks, I highly recommend that you do this oil test and decide - keep, fix it or sell your truck.