If olive oil was used for lamps etc., it makes sense that the modern petroleum industry would have nixed a major market for the stuff--just as petroleum byproducts replaced flaxseed (linseed) for finishing wood. There's no doubt that the olive oil industry is ancient, also no doubt that the fraction of oil produced that gets eaten has gone up as well. As far as how much got eaten... there's difficulty in figuring out just what people are eating now, let alone then.