What did you do to modify the airbox? Also, did you also go with a cat-back pipes with the Flowmaster 40?
Anyway, I noted in a magazine that AEM is coming out with a synthetic filter that takes no oil, and allegedly filters up to one micron. I'm tempted to take a look at that system. I ran Flowmaster two chambers on a 5.0 Mustang about ten years ago, and noted that it could get pretty decent milege when my foot wasn't in it. When the pipes rusted out, I replaced the system with a Dynomax system that touted to be more effecient. By that time I was looking for something that wasn't quite as loud and the Dynomax fit that bill. It just didn't seem like it made as much power with the new mufflers, however.