Law / Driving a Poland's registered car in the EU ? [3]

Hi

I was wondering if there is such a law that limits the period of time in which a polish registered car can be driven in any other EU country. There is however no way to prove the time the car has been introduced to another EU country since there is no border control, so it would sound not possible to me.

I heard of some cases of poles living in another EU country (no Poland) driving a polish registered car which was "borrowed" from a relative. They avoid thus paying car taxes in that other EU country. Is this legal?