Mexicowanderer: I ma certainly no expert but the only way to "clean" the DPF is via a machine or by regens. Switching to ULSD would not help but infrequent usage of the high sulfur fuel should not cause any problems as the DPF will be cleaned with a regen. Just maybe a little sooner. I don't know what happens if you continuously use ULSD. Porbably plugs the DPF. The on vehicle cleaning is done by injecting raw diesel into the filter and lighting it on fire!!!
( A dramatic explanation). When Cat was still building OTR truck engines they had an access door on the DPF and sold a kit one could use to clean the filter.
Moisheh