well...it's various things – like how it's used, under what conditions and I would say that time has something to do with oil changes too. There's not just virgin oil in the oil. There's metal, muck, maybe water (depending upon which oil it is). I'd prefer to tip that out every now and again. Given what came out with the fork oil, I'd change it more often rather than less often – for the sake of a few pounds, well, £20 or £30 pounds or whatever that Ohlins stuff was.