Engine oil, most of which is extracted from petroleum, consist of complex mixtures of hydrocarbons of molecular
weights in the range of 250-1000. Variable amounts of different additives are put into them to inhibit oxidation,
improve the viscosity index, decrease the fluidity point and avoid foaming or settling of solid particles among others.
Terahertz (THz) spectroscopy contains rich physical, chemical, and structural information of the materials. Most
low-frequency vibrational and rotational spectra of many petrochemicals lie in this frequency range. In recent years,
much attention has been paid to the THz spectroscopic studies of petroleum products. In this paper, the optical properties
and spectroscopy of selected kinds of engine oil consisting of shell HELIX 10W-40, Mobilube GX 80W-90, GEELY
ENGINE OIL SG 10W-30, SMA engine oil SG 5W-30, SMA engine oil SG 10W-30, SMA engine oil SG 75W-90 have
been studied by the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.6-2.5 THz. Engine oil with
different viscosities in the terahertz spectrum has certain regularity. In the THz-TDS, with the increase of viscosity, time
delay is greater and with the increase of viscosity, refractive indexes also grow and their rank is extremely regular. The
specific kinds of engine oil can be identified according to their different spectral features in the THz range. The
THz-TDS technology has potentially significant impact on the engine oil analysis.