Panasonic FM has an aqueous electrolyte (same with Panasonic FR), FC doesn't, it uses a different electrolyte solution.
That may explain why FM and FR aren't advertised for automotive use, maybe the electrolyte doesn't like large deviations in temperature. Or maybe Panasonic just didn't bother to certify them, run the tests, pay for the approvals etc?
ps.
FC temp range –55 °C to +105 °C
FM/FR temp range –40 °C to +105 °C