Terahertz time-domain spectroscopy (TDS) transmission analysis as applied to oils typically involves multi-layer sample holders of differing materials. Here we investigate the THz transmission through multilayer systems using a simplified holder design consisting of a milled delrin ring and two thin low-density polyethylene (LDPE) windows. This configuration reduces the complexity of the analysis and increases the accuracy of terahertz time-domain measurements on oil-based samples. We analyze the refractive index and absorption coefficient for different oils. This analysis offers a new approach in monitoring oil quality during manufacturing and degradation with use of oils in food industry. The use of oils that are largely transparent in the THz regime also serves as a medium in which to dissolve chemicals for THz spectroscopic analysis.