Solvent Red 164

Solvent Red 164, also called Oil Red B, is a synthetic red diazo dye. Its chemical structure is 1-[[4-[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 to distinguish low-taxed heating oil from automotive diesel fuel, and by the EPA to mark fuels with higher sulfur content; it is a replacement for Solvent Red 26 with better solubility in hydrocarbons. 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 fluids and some other hydrocarbons, predominantly gasoline.