Solvent Red 164, also called Oil Red B, is a synthetic red diazo dye. Its chemical structure is 1--[phenylazo]-phenyl]azo]-2-naphthol.
The inventors of the product were Morton International under the commercial name Automate Red B.

Its main use is as a fuel dye in the United States of America mandated by the IRS

Internal Revenue Service

The Internal Revenue Service is the revenue service of the United States federal government. The agency is a bureau of the Department of the Treasury, and is under the immediate direction of the Commissioner of Internal Revenue...

 to distinguish low-taxed heating oil

Heating oil

Heating oil, or oil heat, is a low viscosity, flammable liquid petroleum product used as a fuel for furnaces or boilers in buildings. Home heating oil is often abbreviated as HHO...

 from automotive diesel fuel, and by the EPA

United States Environmental Protection Agency

The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 to mark fuels with higher sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 content; it is a replacement for Solvent Red 26

Solvent Red 26

Solvent Red 26, also known as C.I. 26120, is a purplish red synthetic azo dye. Its chemical formula is C25H22N4O, or 1-Solvent Red 26, also known as C.I. 26120, is a purplish red synthetic [[azo dye]]. Its chemical formula is C25H22N4O, or 1-Solvent Red 26, also known as C.I. 26120, is a purplish...

 with better solubility in hydrocarbons. http://www.chevron.com/products/prodserv/fuels/bulletin/diesel/L2_3_9_rf.htm The concentration required by IRS is a spectral equivalent of 3.9 pounds per 1000 barrels, or 11.13 mg/l, of Solvent Red 26 in solid form; the concentrations required by EPA are roughly 5 times lower.

It is also used to dye some hydraulic fluid

Hydraulic fluid

Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...

s and some other hydrocarbons, predominantly gasoline

Gasoline

Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

.