The Mitsu i uses the smart 999cc, 3B21's sibling - the 660cc, 3B20T turbo. (Note, Mitsu "i," not "i MiEV."
Some interesting info can be extrapolated from Mitsu (Eur) and the i. In the case of the 3B21s, Mitsu has reduced the Euro oil change interval from 20,000 km for the 61 ps and 71 ps, normally-aspirated engines to 16,000 km for the 84 ps and 98 ps turbos, a 20% reduction. The 3B20T turbo is pegged at 6,000 miles. If Mitsu also cranked in a 20% reduction in the case of the 3B20T turbo, a "normal" would be 7,500 miles, which, coincidently has been Mitsu's long-time oil change recommendation for their larger cars, along with most other manufacturers.
M-B instituted a paradigm shift in 2004 in the face of mounting owner dissatisfaction with the cost of maintenance, and specified 229.5's long-life oils, and also stretched out the OC interval. smart is in that program. Good or bad? - dunno. Should the '08 smarts have been allowed to go to 10,000 miles on the fossil delivery oil? - ultimately, smartUSA didn't think so, and began what must be a costly and wasteful early oil change program at the VPCs.
I'm guessing that Mitsu was happy, or at least comfortable to see 7,500 miles on their fossil delivery oil.