Why can a reasonably new car consume oil?

Question

I have understood that the oil consumption between cars varies a lot, more than an order of magnitude. And by this I don't mean that a specific model of car consumes more oil than another model of car, but that two identical cars with identical engines and identical miles on the odometer can have different oil consumption, with one consuming 10x as much as the other.

My own personal experience is that I have had three cars long enough to say something about oil consumption: a 1989 Opel Vectra which didn't burn oil but suffered from oil leaks late in its life, a 2011 Toyota Yaris that burned oil but only so little that you didn't have to add oil between oil changes, and a 2016 Toyota RAV4 hybrid with only 3000 km on the odometer for which it is early to say a definite answer about oil consumption but it doesn't appear to be burning it a lot.

Why do some cars consume oil by burning it, even during early parts of their life? Can't the manufacturers consistently build good engines for which the oil consumption is minimal? The acceptable oil consumption specified in the manual, which is usually something like 1L / 1000 km is absurd, meaning you need to add oil over 10 times during a 15 000 km oil change interval.

Of course, a genuinely worn-out engine can easily consume lots of oil, and of course all engines consume some amount of oil. Let's limit the answers to why some relatively new engines consume so much oil that you need to add more oil.

Answer 1

There are several ways a modern engine in running condition can consume oil

1. Past damaged or poor quality piston rings
2. Past gaskets into the coolant system
3. Past gaskets into the engine bay, for example from the valve cover onto the exhaust headers
4. Directly into the intake, via the positive crankcase ventilation system

This list omits obvious problems, like the oil pan having a hole in it from road debris.

Items 1,2, & 3 can be caused by poor maintenance or poor quality parts. I am assuming the engines has been built well and maintained well.

The fourth item is the most likely cause of oil consumption on an otherwise acceptably running engine. The positive crankcase ventilation system allows gases to be removed from the crankcase. This solves a number of problems, mostly the removal of trace combustion gases from the crankcase. Due to emissions regulations in most of the world, these gases must be burnt off instead of ventilating to the atmosphere. As a result, there is a path from the crankcase to the intake of the engine.

The PCV system is explicitly designed not to permit oil from the crankcase from entering the intake. However, not all PCV systems are designed equal. During extended running at high engine RPM, it is possible for oil to be ingested slowly by the engine. This problem tends to self regulate: as the oil level becomes lower due to consumption the rate of consumption also falls. This is caused by poor design of the PCV system or lack of maintenance.

For example, on my Ford F-150 with the 4.6L motor if I do not replace the tiny PCV valve in the valve cover I get oil consumption while driving on the freeway.