Purpose: Increasing flow and/or transparency of oil colour 
Composition: alkyd resin, white spirit 
Qualities:

•    Shortens the drying time of the paint film
•    Increases the durability of the paint film
•    Makes the brush-stroke run, depending on the amount added
•    Hardly influences the degree of gloss
•    Slightly yellowing
•    Can be thinned with white spirit
•    Flammable

reference: www.talens.com