It might be a bit more complicated than that, Mixos. The only rule I know of specifically is Directive 83/182/EEC and that only deals with driving a car registered in one EU country, say Country A, in another EU country, say country B. It specifically applies to residents of country A being able to drive it in country B. That indeed allows you to drive it for 6 months. It wouldn't allow a resident of country B to drive it for six months and, indeed, Greece about ten years ago used that rule to confiscate cars and impose massive fines, often wrongly according to the Commission.
It should be noted that the Directive talks about being resident, not tax resident, so the link mgm referred to is not really relevant. Residence is largely determined by an 185 day rule but with the usual waffle about personal ties etc. You can see a lot of the text here
but for some reason not all of it.
The Directive doesn't say anything about cars registered in non-EU countries, which appears to be mgm's case. This UK document
allows any foreign registered vehicle to be driven in the UK for up to six months, again providing the person driving it is not UK resident. It refers to a Vienna convention in 1968 so it is possible that the same rule applies in Greece but I don't honestly know.
PS I have no idea why mgm is regarded as tax resident in Greece if he lives here for less than 183 days and has no income in Greece. Poor accountant?