What do I look for?

White or black?
Mixed with Olive oil or 100% pure truffle oil?
How do I use? A pour over after cooking?

AFAIK, there is no such thing as 100% truffle oil. It's just bits and pieces of truffles tossed into some oil (mostly olive oil). Try both black and white, as they taste different. Only get small quantities, because it does go rancid rather quickly. Cooking with it is not really recommended as heat degrades the flavors. However, I do use it for making scrambled eggs, so it can be done. No high heat, though. Also use it to drizzle over mashed potatoes, etc. Google has lots of info. Cheers! MM

It's mostly artificial flavorings anyway; I'd generally not bother with it, but
if you must, a couple drops flicked over an otherwise bland dish might help.

Just picked some up in Croatia, and I think the other posters are correct. The truffles are added to olive oil and then filtered out after the oil has picked up the flavors. Black vs white is just a matter of taste. I was recommended not to cook with it, rather add it to a dish at the end.

I was not aware that it goes rancid, so thanks xvimbi for that!

Very few truffle oil offerings have anything artificial whatsoever. The Istria products are much cheaper than the Italian or French ones, usually, but are of generally equal quality, IMHO. We use them for salad dressings and drizzle some over pasta and other foods. They keep well if kept completely in the dark, closed and sealed. Once open none of them last, just as pure olive oil does not.

As several posters have commented the stuff is made by filtering olive oil through truffle pieces, often with a bit of steeping involved. I have never seen one that did not have extra virgin high quality oil.

There is much less difference between white and dark truffle oil than there is between the fresh truffles, but some of the truffles themselves are mislabeled. It is not rare to find cheap, tasteless truffles or truffle oil that repackages truffles from China or somewhere else, using oil from a good location, labeling the entire mess as from Italy or France. I have never heard of a fake from Istria, so we tend to seek the Istrian product because it is consistent.

Just in case you do not know, the truffles of Istria are the same as those from northeastern Italy because the conditions are similar. However, the Istrian product is often in better supply, so is les expensive. If you cannot find them in your area seek out anywhere there are Croatian foods or supplies and you'll probably find them. Otherwise wholesalers that carry Italian products from Trieste (think Illy coffee) often have truffle and olive oil from the region also, often sourced from Istria.

This is purely a personal point of view--I think truffle oil is a huge waste of money. Sure it seems like a much cheaper way to try and infuse truffle flavor to your cooking but it does not compare to real truffle shavings. Restaurants typically use truffle oil to make a dish sound more expensive and increase the markup accordingly.

If you absolutely must have it in your pantry, it should be used as a finish (i.e., drizzled/mixed into your dish after cooking is done otherwise all the flavor/aromatic compounds of the truffle oil will disappear and you'll likely have plain olive oil). If you don't use it fast enough all monounsaturated or polyunsaturated oils should be refrigerated but fats are notorious for picking up odors. Keep in an airtight bottle if you choose to refrigerate.

Nobody should ever think truffle oil is the real thing. It isn't! Fresh truffles from Perigord or Istria are beyond compare. I agree with SOhp that restaurants love to rip people off by using the oil. Still, it does have good uses, but not as a truffle replacement.

Agreed with the recommendations above to not cook with it. One of my favourite uses for white truffle oil is to drizzle it on fresh popcorn in place of butter, and toss with freshly grated parmesan. Delicious

According to the New York Times:

Thanks for posting that antirealist. I was just looking for the same article, you saved me some time.

After reading the first few posts, I chose to buy Truffle Salt rather than the oil.

It has a very earthy flavor and paired well with risotto that we made last weekend. We just use it as a finishing salt.