I have seen many types of olive oil at the store: extra virgin, light olive oil, etc. I know that extra virgin is the first cold pressing of the olives and any derivation of oil after that comes from re-pressing the olive under different conditions, but what effect can this have on the quality of oil? Does it change the healthfulness of the oil?
This link seems to have a good definition for each type: New US Olive Oil Standards
The USDA recently announced that, for the first time since the 1940s, it is revising standards for olive oil grown and imported in the US. Labels such as "virgin" and "extra virgin" must now be scientifically verifiable, and mostly meaningless terms like "light" will be done away with. The new regulations, scheduled to go into effect on October 25, 2010, are in line with quality standards already adopted by the California Olive Oil Council (COOC) and International Olive Council (IOC). The new grades will include:
U.S. Extra Virgin Olive Oil: Virgin olive oil which has excellent flavor and odor (median of defects equal to zero and median of fruitiness greater than zero) and a free fatty acid content, expressed as oleic acid, of not more than 0.8 grams per 100 grams.
U.S. Virgin Olive Oil: Virgin olive oil which has reasonably good flavor and odor (median of defects between zero and 2.5 and median of fruitiness greater than zero) and a free fatty acid content, expressed as oleic acid, of not more than 2.0 grams per 100 grams.
U.S. Olive Oil: Oil consisting of a blend of refined olive oil and virgin olive oils fit for consumption without further processing. It has a free fatty acid content, expressed as oleic acid, of not more than 1.0 gram per 100 grams, and has acceptable odor and flavor characteristic of "virgin olive oil."
U.S. Refined Olive Oil: Oil obtained from virgin olive oils by refining methods that do not lead to alterations in the initial glyceridic structure (basic glycerin-fatty acid structure). It has a free fatty acid content, expressed as oleic acid, of not more than 0.3 grams per 100 grams, and is flavorless and odorless.
Independent of processing, the cultivar of olive is also key to a healthful and Paleo-friendly oil. Some olive varieties are sky high in undesirable omega 6 PUFA (~23g/100g), whilst others are much lower (~3g/100g). If you are monitoring your PUFA intake, remember that online fatty acid breakdowns are often composites or averages.
EDIT: So far the best that I have found is the Kalamata varietal; high in O3 (close to 1g) with just 5g of O6. I like Gaea Kalamata DOP Extra Virgin Olive Oil, which is cold pressed, organic, carbon neutral/ethically produced and pretty cheap (£3.99 for a 500ml bottle). It's also absolutely delicious. :)