I had one of the H2O vacs before the Turbo and didn't have a problem with using Rainbow scents. If you're really thrifty I know someone who uses Vanilla or Almond Extract in her Rainbow.
essential oil would be perfect. I use essential oil in my rainbow or sirena all the time. I like it because there is zero foam and it smells awesome. Eucalyptus, lemon, orange are my favs. For some reason, the pine essential oil does NOT smell good to me, neither does Tea tree oil.
Got some cheapo "orange oil" at supermarket. Put just a few drops in and it worked very well! Now I wonder if I can let it run on low speed to "freshen the air"?