Parts Washer, What Solvent?

Just bought a 20 gallon parts washer. I'm looking for recommendations on what solvent to use. It holds 12 gallons. They want about $40 for a 5 gallon can of solvent at the store where I bought the washer. The instructions say you can use Diesel (LOTS cheaper than $8/gal. for the stuff above) but the sticker inside of the lid specifically says not to use diesel. (WTF?) or other flammable solvents. Is this just a CYA type of deal for the mfr? What about Kerosene? I don't smoke & hope not to set the thing on fire. What works for you guys?