Use the foam. That keeps the paper filter cleaner and you can wash the foam outer cover.
Thanks Skip. One more question for you. The maintenance kit I ordered from Ziller arrived yesterday. The new airfilter came with a foam skirt that surrounds the pleated filter. The OEM air filter set up on my unit did not have the "skirt". Is this some "improvement" that I should install, or should I use the OEM configuration.