From Waste to Lubrication Resource: A Systematic Review of Waste Cooking Oil-Based Greases and Liquid Lubricants

Waste cooking oil (WCO) is generated globally in large quantities, and improper disposal contributes to significant environmental problems. Recently, WCO has attracted increasing attention as a sustainable base fluid for lubricating grease due to its biodegradability, low cost, and favorable physicochemical properties after thermal degradation. This study presents a systematic review conducted with reference to the PRISMA 2020 guidelines of WCO-based grease and lubrication systems published between 2000 and 2025. Scopus was systematically searched, resulting in 22 peer-reviewed studies meeting the inclusion criteria. The review shows that thermal degradation increases WCO viscosity, polarity, and the relative proportion of saturated fatty acids, thereby enhancing boundary lubrication behavior. Tribological performance was found to depend more strongly on formulation strategy than feedstock variability, provided that appropriate pre-treatment is applied. Optimized WCO-based greases achieved coefficient of friction (COF) values as low as 0.0253 and wear scar diameters (WSD) of 467 µm, demonstrating performance comparable to conventional mineral oil greases. Non-soap thickeners exhibited thermal stability exceeding 350 °C, while additives such as molybdenum disulfide (MoS2) improved friction and wear performance. Overall, this review establishes a structure–property–performance framework linking thermal degradation chemistry, formulation design, and tribological behavior in WCO-based lubrication systems while highlighting challenges related to standardization, long-term stability, and industrial validation.