Had a customer call up complaining that there rental pump was using a quart of oil every 12 hours. Engine had an 8 gallon oil capacity, and was an old detroit diesel. Engine had to drop a gallon before it got to the add mark on the dip stick. I told them not to top it up at the 7pm check, and to check the oil when they came in, in the morning. Oil level never dropped below the one quart low spot on the dip stick. Told them to not worry about it being 1 quart low and to run the snot out of it
Some of those older engines would burn oil if it was at thel full mark on the dip stick, but would stop once it got a quart or two down.