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There are really only four significantly different finishes that major marketers call “oil”:

Straight oil. There are two main varieties of straight oil - tung oil and linseed oil. Tung oil is usually found labeled as “pure” oil or “China wood oil”. There are 2 forms of linseed oil, raw and boiled.

Polymerized oil. There are 2 principal kinds of this type of oil also. You guessed it, polymerized tung oil and polymerized linseed oil. Polymerized oils perform more like varnish than straight oil. They are also expensive and can be hard to find.

Regular varnish (including polyurethane) that has been thinned with mineral spirits. This finish is often sold as oil but is technically a wiping varnish.

A blend of straight oil and varnish (also including polyurethane). This oil/varnish mix is often sold as Danish oil.

Take a look at some of the qualities of each of these major categories.

Linseed oil is extracted from seeds of the flax plant. Metallic driers are added to the oil to speed drying time. The linseed oil mix that is available to consumers cures in about a day. Of all finishes except wax, linseed oil is the least protective.

Tung oil is extracted from the nuts of the tung tree. Many high-quality varnishes are made with tung oil, but it is rarely used as a finish on its own. Tung oil can be fairly water-resistant with five or six coats, but is too soft to provide superior scratch resistance. In fact, tung oil will require five to six coats to give a pleasing smooth appearance and cures very slowly. The only advantage tung oil has over linseed oil other than water resistance is the fact that it will yellow considerably less. This can be important on blonde woods that you don’t want to darken.

Polymerized oil can be made by heating any curing or semi-curing oil in an oxygen-free environment to around 500 degrees Fahrenheit. This increases its hardness and reduces its curing time. This oil is commonly used in ink and outdoor paint. This oil cures very fast and very hard and resists water well also. It is often used in high-end gun stocks. Two problems with this oil are its cost and the fact that it cures too fast to be applied and wiped off unless it is thinned greatly with mineral spirits.

Wiping varnishes include most of the “oil” finishes found on store shelves today. They are not in fact oil at all, they’re varnish. There is a difference. Varnish cures much faster than oil. It also cures glossy (unless flattening agents are added by the manufacturer) and is hard when cured. This hardness is the most important difference, as it allows you to build up a thicker layer of finish with repeated coats.

Oil/varnish blends are simply what the name implies. Oil and varnish (including polyurethane) are compatible so they can be mixed. This combines the properties of both, reducing the gloss and increasing the cure time, making it easier to apply than varnish, while still providing the hard cure that varnish is used for.

Hopefully this list will help you make the right decision on the finish to use on your wood project!