On my '93 a few thousand miles after I bought it I swapped out the filter. Bright green pond scum looking algae when I poured it out onto the white cardboard filter box. As a former fuel hauler I did see a 1000gl tank at Marathon Bordeaux in Nashville labeled "algaecide." Allegedly biodiesel will clean all of the trash out of your tank/lines and dump it into the filters if you have been running straight diesel.