Viscosity is an important quality specification for residual fuel oil. Specifically, it is a measure of a liquid's resistance to flow. High viscosity fuel oil is more difficult to pump and therefore less desirable as a product.

Refiners manage fuel oil viscosity either through processing or through blending.

Visbreaking- the visbreaker is a process unit designed primarily to reduce the viscosity of resid by thermally cracking it

Blending - the viscosity of resid can be reduced by blending it with a lower viscosity "cutter stock". This is typically either cycle oil or slurry from the FCC, or straight run kerosene