I never trust use-by dates on things as I know food companies put them on to make us buy things we don't need (cynic, moi?), BUT - wondering how long cooking oils last once opened. In theory I have a rather good selection (hazelnut oil, avocado oil, chilli oil, garlic oil, sesame oil etc.), but have a nasty feeling probably needs chucking out. Can anyone throw some light on the matter?