I've looked but haven't found a torque spec for the oil drain plug in any of the Fiat workshop manuals. Generally you just nip them up, no need to go superman on them.
But for a plug with a copper or a crush sealing washer I'd tighten to about 25-30 ft-lbs. As an example, Toyota, which uses a crush washer, recommends 27 ft-lbs. on a steel oil pan.
Hth,
Al.