Does all cooking oil, solidify, doesn't vegetable oil remain liquid even at room temperature?

The only reason I ask was that one of our technicians scrounged a few litres of the stuff from our canteen, the used stuff of course, filtered it and poured it straight into his tank. I think the ratio was 1:15 or 1:20 I can't remember exactly and the guy's moved on now so I can't ask.

What was noted was a huge cloud of black smoke, I mean most diesels give off smoke, but you could always tell this one because he very nearly had to keep his lights on all the time the cloud was so thick. Strangely enough the smell wasn't as strong, as it would be if it were just of diesel.