Same subject, different question.

I've recently moved to San Luis Potosi and have my FM-3. My license plates are expiring in June, and I'm having trouble getting them renewed because I don't have a residence in the state. My car is only 4 years old, so registering it here would be costly.

A friend here said I don't need to have the car registered unless I plan on driving back into the US, which I don't. Is this true? Would I be okay on expired plates?