Oil is not shelf stable. It can go rancid fairly quickly.

It seems to be stable on my shelf...

...I tend to buy very large bottles of olive oil for instance and they don't degrade noticably over six months or more. Things like sesame oil do definitely go off relatively quickly when exposed to air though.

By the way, the xanthan gum is probably used as a stabilizer to help prevent the vinegar and oil from separating too quickly, and not as a preservative, in case you didn't know what that was.

They use it in the UK as well, horrible stuff. You can always spot a sauce or dressing that has it in it, because they have the nasty overly-shiny appearance.

Quite why salad dressing needs corn syrup added to it I don't know.