I honestly think you're all worrying
way too much. If Fiat say it's 18k or 1 year, then that's what it is! It doesn't need anything else. I will be running mine to 100k in two years, and I will only do the standard services, as required by the service schedule. I'm not in the slightest bit worried that this isn't enough.
I ran a citroen that had 20k service intervals for three years, and 155,000 miles, and the engine just got "normal" service from the citroen garage, and it was fine. At 155k it ran just as well as it did at 10k , if not better
the technology has been in place for a while, not to need an oil change for 100k, but car makers are unlikely to do this, as it would eat into their profits... I believe I read
years ago that honda had this technology running, and successfully tested, after all the billions they spend on R&D if they gave me a car and said "it doesn't need an oil change for 100k, just top up with the right stuff when ness" I would believe them, why wouldn't I? lol...