From Google: See :
http://www.naturalhandyman.com/iip/infpai/inflin.html
"Boiled" linseed oil is not boiled. The actual boiling of some oils changes their drying characteristics, true. With linseed oil, though, it is the addition of certain solvents that causes linseed oil to dry more quickly, acting as if it were boiled. This makes it a better product for preserving tool handles, decks, and furniture. I suppose they should have named it "sort-of-boiled linseed oil", or "kinda-like-boiled-but-not-really-boiled linseed oil". You know... to make it less confusing. Uh huh.
Why shouldn't you pick linseed oil as your first choice in a preservative?
Linseed oil has some negatives. Granted, it has been used with some success for many years. But welcome to the modern world! Today, there are scads of linseed oil-based paints and preservatives that expand on linseed oil's good qualities while overcoming (at least to some degree) its drawbacks.
Some of the problems with straight linseed oil, boiled or raw, are:
Sometimes linseed oil can take forever to dry... or stays sticky or doesn't dry at all!! This is a nightmare situation that happens too often when linseed oil is applied either (1) too thickly, (2) onto damp materials or (3) when the temperature is too cold. Thinning linseed oil with turpentine can help somewhat, but even with thinning it is important to apply thin, multiple coats but allow each coat to dry before applying the next!
No UV (ultraviolet) light resistance... UV causes more damage to exposed wood than any other factor, destroying wood fibers and setting it up for attack by mildew, fungus, and insects.
Linseed oil is mildew food... Many vegetable oils are food products for humans... all vegetable oils are food products for mildew! Linseed oil is not completely denatured, so it can encourage rather than discourage mildew growth.
Linseed oil does not harden sufficiently to offer enough resistance to abrasion to be a suitable deck floor preservative... at least by today's standards. Linseed oil has been used for interior wood floors, but it must be waxed for durability! Waxing an outside deck would be dangerous, even if you hang a "Slippery When Wet" sign!
Difficult to remove from wood... Multiple coats of linseed oil are gummy and difficult to remove fully for refinishing.