In Denver, because of our air pollution problems, a cleaner burning fuel is sold in the wintertime. I always made a point of making sure I had that fuel in the tank when I went for an emissions test. An experienced tech or engineer could tell you whether such a fuel would have any effect on NO readings.
I also religiously change the oil and air filters before an emissions test but I think the oil change only affected the hydrocarbon readings.
Your engine should be hot befoe taking the test. In the UK, they measure the temperature of the oil before the test.
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13