Used about everything...diesel, various lightweight oils,etc.., but by far and away the best is good ole' concrete form release oil or Corvus oil. Diesel works, but doesn't make nearly as nice of a smoke trail and doesn't stay nice and thick with a good "hang time".
I bought a barrell of concret form release oil, costs more than diesel but cheaper than branded smoke oil. Plus...it smells better than the Hyd/Diesels! We get together up here in MN and a few of us order up a few barrells at once to get the price down.
Here's a quick clip with the smoke on between 1/4-1/2 output with the form release oil.....
I use Superdri in my Extra but make sure you put a Radiator on your credit card to cool it down before you order that. I think my last barrel was $404 plus shipping.
Steve