Terahertz-time-domain spectroscopy (THz-TDS) in the range of 0.5–2.0 THz was evaluated for distinguishing among gasoline engine oils of three different grades (SAE 5W-20, 10W-40, and 20W-50) from the same manufacturer. Absorption coefficient showed limited potential and only distinguished (p < 0.05) the 20W-50 grade from the other two grades in the 1.7-2.0 THz range. Refractive index data demonstrated relatively flat and consistently spaced curves for the three oil grades. ANOVA results confirmed a highly significant difference (p < 0.0001) in refractive index among each of the three oils across the 0.5 - 2.0 THz range. Linear regression was applied to refractive index data at 0.25 THz intervals from 0.5 – 2.0 THz to predict kinematic viscosity. All seven linear regression models, intercepts, and refractive index coefficients were highly significant (p < 0.0001). All models had a similar fit with R2 ranging from 0.9773 to 0.9827 and RMSE ranging from 6.33 to 7.75.The refractive indices at 1.25 THz produced the best fit. The refractive indices of these oil samples were promising for identification and distinction of oil grades.