Chinese cooking usually calls for vegetable oil like canola or sunflower seed oil because the oil itself doesn’t carry much flavour. It’s not to say you can’t use butter or olive oil but you can definitely taste the difference. Oil is not only used for stir-fry or deep-fry, but is sometimes called for to marinate the meat too. The function is actually not to add additional flavour but to help coat the meat so it doesn’t dry out as easily during fast cooking, such as stir-frying.