Looking for help on the forums since the dealer cant find a leak.

I have a '04 allroad with a 4.2L V8 with 81k miles. The problem I am having is that I am not only adding oil between service intervals but on a recent trip I had to add 1.5 qts in less than 300miles. There are no apparent leaks and the dealer couldnt find anything to indicate a problem. Any ideas?

Thanks