I have used 中石油 not "中石化" for more than 2 years. Before that, I also used ESSO as well.
From my experience, 中石油 Cheaper-oil is so far so good. My car has not encountered any problem. For sure, my car is low-grade Toyota Corolla. There should not have any performance issue. About 中石油貴油, I used for almost half year before switching to other provider. In these few months, 1 tank of gasoline can run about 0.2 to 0.3 km/L more. That's why I kept on using higher grade gasoline.
Once I got a Shell -1.9/L card, I switched to Shell V-power. For oil consumption, there is not much different. But, my car has a little bit better response ... like people said "好爆D".