Nothing to worry about. Plenty of FIATians stick to 15,000km intervals and are happy with it. (Me excluded ). Check with your TASS if they allow anything other than Selenia. The viscosity of an oil does not necessarily tell us about it being fully synthetic or not. The Selenia that we have in India is Semi Synthetic 5W-40. However, oils like Castrol Edge 5W40, Mobil D1 5W40 are fully synth. Cheers.