<p>In this paper, we present the design and theoretical analysis of a rectangular core photonic crystal fiber (RC-PCF)-based sensor model specifically modified for petrochemical oil detection. A finite element method-based COMSOL Multiphysics software is used to analyze several optical properties in order to assess the effectiveness of this model. In this investigation, many analytes, including kerosene, diesel, and petrol have been taken into consideration for sensing purposes. Various analytes are introduced into the core hole through infusion. The optical parameters of the sensor are evaluated by simulating the terahertz frequency range from 0.4 to 4 THz. At an operating frequency of 1.50 THz, the suggested design shows high relative sensitivities of 99.88% for kerosene, 99.99% for diesel and 99.61% for petrol. The suggested model exhibits significantly low confinement loss values, which are 7.0 × 10⁻¹² dB/m, 3.48 × 10⁻¹¹ dB/m and 3.93 × 10⁻¹¹ dB/m for kerosene, diesel and petrol. Additionally, the effective mode area of the designed PCF is also exceptionally low for all analytes, measuring 4.90 × 10⁻⁰⁸ m² for kerosene, 4.88 × 10⁻⁰⁸ m² for diesel and 4.91 × 10⁻⁰⁸ m² for petrol. Furthermore, within the specified range, the PCF shows a numerical aperture (NA) at 1.5 THz. At 1.50 THz, the NA values for kerosene, diesel and petrol are 0.45, 0.45 and 0.453, respectively. Consequently, the birefringence values of 2.70 × 10⁻⁰³ for kerosene, 2.70 × 10⁻⁰³ for diesel and 2.65 × 10⁻⁰³ for petrol is obtained from the ideal profile. The proposed sensor can be produced with 3D printing and extrusion technologies. The improved detection capabilities make it an important feature of useful oil sensing systems, especially in industrial environments.</p>