A novel surface plasmon resonance (SPR)-based gold-coated photonic crystal fiber (PCF) biosensor is proposed herein for the detection of oil adulteration. The overall performance is studied using full three-dimensional (3D) simulations based on the finite-difference method on a mesh of 221,209 elements in COMSOL Multiphysics software, considering adulteration measurements at 25 °C (298 K) on different mixtures and concentrations of adulterated oil. Such measurements of fuel adulteration can have important implications for the global economy and gross domestic product (GDP) growth, as well as for quality control of imported or exported crude oil and fossil fuels worldwide. In this regard, the oil sensor proposed herein represents an important option and is shown to offer better performance compared with other sensors. The main advantages of such plasmonic sensors are their reusability, low cost, and portability, while offering better accuracy than existing electronic sensors. The maximum performance in terms of the major optical parameters of birefringence (B_i), coupling length (L_c), power fraction (P_f), confinement loss (α_c), amplitude sensitivity (S_a), wavelength sensitivity (S_w), resolution (Rl), transmittance (T_x), transmittance variance (T_v), relative sensitivity (S_r), figure of merit, and resonance (R²) is found to be 4.0 × 10⁻³, 39 µm, 45%, 700–1120 dB/cm, −9750 RIU⁻¹, 52,941.17 nm/RIU, 9.32 × 10⁻⁴ RIU, 178 dB, 210 dB/RIU, 93.5%, 1210, and 0.97029, respectively, confirming that the SPR-PCF sensor is the first appropriate option for measurement of the adulteration of fossil fuels or crude oils.