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I'm concerned by how difficult it is to learn exactly what kind of linseed oil we may actually be dealing with in any given situation, marketed as it is in varying qualities and types with equivocal or conflicting descriptions.
As with olive oil, the extraction process results in grades of differing "purity", with different properties. The "gold standard" is cold-pressed (extra virgin?); the seeds are simply smashed to yield free oil. The press tailings (and whole seeds) may also be tapped for the oil they contain by running them through a centrifuge-like machine called an "expeller". Adding steam heat to the process allows the extraction of still more oil. Once refined, it's a moot point whether "cold pressed" oil is actually superior for our purposes.
Cold-pressed linseed oil goes rancid unless refrigerated. In order to prevent it from going rancid, it has to be refined; waxy solids must be removed, similar to the way milk solids are removed from clarified butter. Refining either butter or linseed oil stabilizes them so they can be stored at room temperature without going rancid.
Alkali refined linseed oil is produced by spraying a dilute alkali solution on the surface of a vat of oil. As water is heavier than oil, it slowly settles to the bottom. The alkaline solution saponifies in contact with the oil, causing an ion exchange which attracts foreign matter, which is carried along with the water (which is now a soap) to the bottom. It's a complex process; in spite of the alkaline treatment, the oil retains varying degrees of acidity, which is necessary to form tough paint films during the polymerization which occurs as the oil oxidizes (dries).
The most carefully graded and refined linseed oil is used in the manufacture of printers' inks. SP uses this grade of oil in their paint.
In the "good old days" before petrochemicals eradicated linseed oil along with turpentine and natural resin varnishes from paint store shelves, Spencer-Kellogg linseed oil, cheap by the gallon at the local farm Co-op store met all reasonable standards for use in oil painting. So far, the stuff currently available to me (Startex) appears to be OK, but each new lot I buy will require testing, as the quality of all painting materials (regardless of their source) has become a moving target.
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