I don't think he means lighter fluid as in the ronson stuff for Zippo's. I think he means the slim butane cans with a tip that seems just right for airsoft.
It's not particularly dangerous as butane is as flammable as propane, but butane only exerts 40ish psi whereas propane pushes 115psi at room temp. 40psi is too low to cycle most gbb slides. You'd get a slide lockup and the gun would poof all of it's gass on the 2nd or 3rd shot.
Want nearly free GBB gas?