A liquid-loaded frequency tunable cavity bandpass filter (BPF) is presented. A dielectric fluidic material, dimethyl silicone oil (DSO) with excellent thermophysical characteristics (working temperature from -50 °C to 180 °C) and extremely low loss tangent is employed as a dielectric loading. The frequency reconfigurability of the proposed design is realized by altering the liquid level inside the cavity resonator. The filter achieves a wide frequency tuning range as well as a high Q factor. Moreover, this design shows significantly improved environmental suitability in extreme temperature cases, outperforming the existing microfluidic-based RF devices using water or liquid metals. A four-pole tunable cavity bandpass filter is designed and verified. A cross-coupling structure comprising a metal loop structure is used to introduce transmission zeros in the proposed filter, which enhances the skirt selectivity and out-of-band rejections. We demonstrate that the center frequency of the proposed BPF can be tuned from 4.92 GHz to 6.16 GHz, and the filter achieves a high Q factor between 521 and 1527. The measured results agree well with simulated results.