Exactly same when I got the car serviced at a independent Jag specialist. The bar was in the middle and said OK. I queried the specialist and told him the bar is in the middle and should I be filling it up? He said it was normal for it to show the mid bar and wouldn't recommend topping it up unless otherwise the display suggested.Whenever I had my XF Dealer serviced it came back with the oil level gauge bang on half way which I believe is correct. It would also bring your attention to any diesel dilution if the level went up.