"When the oil level goes down, it has either been burned inside the engine or it has leaked past a gasket or seal, says Jim Kerr. Determining how the oil disappears when there are no external leaks may seem complicated but it is really very simple, he says.

Some engines use more oil than others, but how much is too much? Leaf through the repair manuals of several auto manufacturers and you will find a variety of specifications. One states that the engine should use no more than 1 quart (.946 litres) of oil in 3200 km. Another states a litre in 500 km. This wide range of specifications challenges anyone to determine what is excessive. A lot has to do with the design of engine, the application, the oil viscosity and how it is operated."

Now I am no expert on KM but I beleive it is 6/10ths of a mile. Now a liter in 500 km is more than 1 quart per 300 miles. More in the article state more commercial engines have more consumption due to less tight seals. I wonder if Isuzus commercial truck experience has allowed them to be a bit more liberal than others?