OK I didn't consider oil vapours as something that would affect the O2's, that's possible. But really, is it that much?

In my case installing an orifice didn't change the fuel trims nor the idle quality, but it may depend on the engine.

I understand the PCV valve is important in a carburated car, where the idle quality is affected by "false air" coming from the crankcase. In this case it's good to have a PCV valve that closes completely at low manifold pressures.
But with a car with an idle pintle... I think an orifice should be good enough. The amount of false air is compensated by the idle pintle (assuming the idle relearn procedure is correct).

What is the reason for GM to install an orifice in certain truck engines?