+1. This really isn't rocket science- people are running rather large risks for the sake of saving a few quid every (if you follow the guidance
) 18000 miles. Just follow the sodding specification properly.
I would certainly follow FIAT's recommendations (note I did not say "specifications" there)
to the letter whilst in warranty- buy Selenia from shop4parts if you're on an "as cheap as possible" basis (why did you buy a 500 if you were that bothered?). An extra (say) £50 in oil over the warranty period is money well spent, compared to arguing the toss with FIAT (who aren't exactly renowned for their generous warranty provisions) over a Uniair module failure.
I'd also strongly disagree with the "a modern engine cares less about oil specifications than an older engine" comments above.