Petrol is a mineral-based oil painting medium used in the production of oil works to adjust the viscosity and concentration of the paint and to dilute dry oil. 

Painting mediums used in oil colors comprise mainly two types: volatile and dry oil. Petrol is classified as volatile oil and most of the contents evaporate into the atmosphere after use. It also dissolves gum and resin. It is a mineral-based volatile oil made by distilling and refining crude oil to produce a blend of fatty and aromatic hydrocarbons, which are its main components. It evaporates and dries slightly more slowly than turpentine and has less dissolving power but is less prone to oxidation and yellowing, and its smells less acrid. Recently, odorless petrol, which is virtually odor free, is also available but it has even less dissolving power. 

Petrol is used mainly in the early stages of the oil painting production process such as when doing preliminary sketches and starting to paint. To use, add a solution of dry oil diluted with petrol to a font or some other container, dip the tip of a brush into the solution when required, adjust to the required viscosity and concentration by combining this with the oil paint on the brush tip and then begin painting. The solution will have more petrol and less dry oil when the painting begins but the ratio of petrol will gradually reduce as the work continues. Other uses also include use as an agent to dissolve gum (Dammar gum, etc.) when making your own blended oil or varnish but because it has less dissolving power than turpentine, it could result in making the solution cloudy or cause precipitation. 

Note that petrol contains no ingredients that will fix the paint to the support and so overuse can degrade the durability and fixing capacity of the painting or detract from the gloss of a color work piece. Because of its volatility and flammability, interior environments should be well ventilated and the petrol should be kept well away from naked flames. When stored, oxidization can produce a gummy, sticky solution and yellowness. Make sure the lid is closed firmly and keep away from hot environments. 
Petrol can be purchased at general art supply stores.