This study investigates the potential use of the organic fraction of oily sludge (OFOS) as a partial or complete substitute for the commercial petroleum-based processing oil PN-6Sh in rubber compounds based on synthetic isoprene rubber (SKI-3). OFOS was recovered from oily sludge by liquid extraction. The chemical characteristics of the original oily sludge, OFOS, and PN-6Sh were evaluated using gas chromatography–mass spectrometry (GC-MS), Fourier-transform infrared (FTIR) spectroscopy, and hydrocarbon group composition analysis. Hydrocarbon-group analysis showed closely matched light- and medium-aromatic fractions in OFOS and PN-6Sh, whereas GC-MS and FTIR were used as complementary qualitative characterization methods. Five formulations were investigated with a total processing-oil content of 6 phr and PN-6Sh replacement levels of 0–100%. Increasing OFOS content caused only a moderate change in Mooney viscosity, while the mechanical properties of the vulcanizates remained nearly unchanged throughout the replacement range. At 100% replacement, the elongation at break after thermal aging was 317%, identical to that of the control vulcanizate. Under the investigated laboratory conditions, complete replacement of PN-6Sh with OFOS preserved the evaluated mechanical properties and thermal-aging behavior without a practically meaningful loss in performance. Further application-specific qualification is required before industrial equivalence can be established.