tailpipe sniffer test? - heavier > lighter oil, less chance of burn off, and no to synthetic, the molecules are "cracked" and has a higher chance of leaking into the cylinder --- atleast 3/4+ tank of gas, full is ideal... less chance of failing due to failed vapor system.
i think if you fail the first test, the second one is usually free. if you don't pass the first one you might wanna try that "guranteed to pass (in a bottle)" stuff. i heard it's mostly alcohol or something and basically works by just getting your cat super hot so that it works better.
also the best way to heat up the cat beforehand is to have a good drive on the highway before you go get it tested.