I will say that Costco has the best olive oil, (at a reasonable price point) we have ever tried. We use it daily and it has both a fresh flavor and a slightly peppery aroma.
Family farms in Italy charge roughly $50 per liter for estate cold-pressed extra virgin.
Costco olive oil is $11 per liter. I just bought one -- haven't opened it yet. But I seriously doubt it is 100% unadulterated cold press olive oil. I certainly wouldn't "bet the farm." In fact, if I had to bet the farm, I'd bet on at least a little funny business going on. I don't have any other options here.
Italian oil -- particularly Tuscan oil -- is the most suspect. Tuscany accounts for 2-3% of Italy's total production. Italy is the second-largest producer in the world -- around 20% of the world total. But they are selling more oil than they produce. A great deal more. This is because Italian law says any oil bottled in Italy can be labeled "Italian." So much Italian oil is actually Tunisian or Moroccan.
If that was the worst form of cheating, I wouldn't care and I certainly wouldn't be typing about it. There is nothing wrong with North African olive oils. They're quite good.
Here are some other cheats:
Blending with spoiled/rancid oil which has been chemically scrubbed. This is how producers get rid of oil they
should send to a diesel refinery.
Blending with cheaper vegetable oils.
There is even oil that has no olives whatsoever -- just cheap vegetable oil and chlorophyll for flavor and color.
Who is doing this? Organized crime and terrorist groups. The profit margin on bogus oil is right up there with cocaine. So not only are people being ripped off, the profits are funding misery.
Without a lab test, there is no way to be 100% sure. But just assume that any inexpensive oil is at best a blend. You can't be 100% sure about any expensive oil, either. The best way to make sure you get what you pay for is to find a farmer and buy direct.
Sadly, there are dozens and dozens of foods that have similar problems. Olive Oil is just the most profitable.