My friend left 中石油 in 1996 becuse he got only about 300 yuan/month at that time while his classmate got at least 500 yuan/month outside 中石油. The most important thing was he couldn't see any future while stayed in the office without anything to do. Two years ago when I was back in China he told me a lot of his friends who left 中石油 10 years ago went back to 中石油 again becase 中石油 expanding so quick and they had the priority being picked but never got their preview positions.