Lubricating Oils, Petroleum, Hydrotreated Bright Stock

Lubricating oils, petroleum, C>25, hydrotreated bright stock-based (CAS 72623-83-7) is a severely hydrotreated, high-viscosity residual lubricating base oil fraction derived from crude oil refining. It is characterized by a very high kinematic viscosity (typically 90–150+ cSt at 100°C) and a high flash point (>200°C), making it resistant to volatilization at engine operating temperatures.

In engine oil additive formulations, bright stock serves as a viscosity modifier and film-strength improver — its primary function is increasing the oil film thickness under high-pressure, high-temperature boundary lubrication conditions. This is a physical mechanism (viscosity) rather than a tribochemical one (as with ZDDP anti-wear additives). Bright stock is also used to reduce oil consumption in worn engines by increasing viscosity and improving ring-to-cylinder seal effectiveness through film thickness.

The ingredient carries H304 (aspiration hazard, Category 1) at the ingredient level due to petroleum origin, but this classification is routinely waived at the product/mixture level under the CLP viscosity exemption rule: substances with kinematic viscosity ≥ 20.5 mm²/s at 40°C are exempt from mixture-level H304 classification. Bright stock at 110 cSt (100°C) — which translates to approximately 300–500 cSt at 40°C — is far above this threshold.