I discovered a leaking intake manifold gasket by using a trick Tone posted a while back: with the engine running, spray some starting fluid around the edges of the gasket (in this case, where the intake manifold attaches). If the engine revvs up, it's sucking in the starting fluid, indicating where the gasket has failed. Replacing my intake gasket has improved my idle, gas mileage, and for the most part eliminated my oil loss. My loss was about a quart every 1000 miles and although there was some oil on my valve covers, there wasn't enough to directly implicate the intake gasket (I thought most of it was coming from the PCV valve... wrong!). The starting fluid left little doubt in my case. I'm not sure this method would work if it's a leaky valve cover gasket, but it's an easy way to eliminate other potential sources of oil loss. It works for sure on any gaskets on the intake (intake manifold, common chamber, etc).