We investigated the thermal transport properties of two α−RuCl3 crystals with different degrees of stacking disorder to understand the origin of the previously reported oscillatory feature in the field dependence of thermal conductivity. Crystal I shows only one magnetic order around 13 K, which is near the highest TN for α−RuCl3 with stacking faults. Crystal II has less stacking disorder, with a dominant heat capacity at 7.6 K along with weak anomalies at 10 and 13 K. In the temperature and field dependence of thermal conductivity, no obvious anomaly was observed to be associated with the magnetic order around 13 K for either crystal or around 10 K for crystal II. Crystal II showed clear oscillations in the field dependence of thermal conductivity, while crystal I did not. For crystal I, an L-shaped region in the temperature-field space was observed where thermal Hall conductivity κxy/T is within ±20% of the half quantized thermal Hall conductivity κHQ/T, while for crystal II, κxy/T reaches κHQ/T only in the high field and high temperature regime with no indication of a plateau at κHQ/T. Our thermal conductivity data suggest the oscillatory features are inherent to the zigzag ordered phase with TN near 7 K. Our planar thermal Hall effect measurements suggest the sensitivity of this phenomena to stacking disorder. Overall, our results highlight the importance of understanding and controlling crystallographic disorder for obtaining and interpreting intrinsic thermal transport properties in α−RuCl3.