My understanding is that since the ICE is coming on fairly frequently for short(ish) periods you might need more frequent oil changes than you think. I try to follow something inbetween the recommended "regular" and "severe" intervals listed in the manual.
Given the lower quality of NA engine oil - this isn't dig at you, but has been acknowledged over the years in numerous posts by your compatriots - it seems foolish to penny pinch on servicing when you have invested $40k+ in buying the car. The ICE is the most expensive item after the battery and like all mechanical components is liable to wear and tear. Mitsu would justifiably invalidate your warranty if you don't adhere to the servicing regime - which could get very expensive!