Did my first oil change on my smart a few days ago and had to use my somewhat awkward Vise Grip Chain-Type tool to get a grip on the filter. Had intended to use a cap wrench and a socket wrench driver to do the job, but of course none of my existing cap wrenches fit the Mitsubishi filter. Went out afterwards and did some searching for the proper cap wrench and found the following:
The cap wrench is available from Pep Boys and is a RiteFit A256, $4.99 + tx. The filter is usable with a 3/8” drive socket wrench. The caps are manufactured in China (where else!) and are distributed by CTA Manufacturing Corp., Carlstadt, NJ 07072. The web address is
CTA Automotive Specialty Tools.
Hope this is of some help.