There's a product called Clenzoil which won't show the rainbow effect, and another one called RIG.(not to be confused with the above mentioned RIG grease)
I've used both, and prefer the Clenzoil.
Be forewarned though, that they are for appearance only. They offer little in the way of protecting the finish.
I picked up the Clenzoil at a local show after talking to one of the guys that was a regular seller at most of the local shows. I asked him why his guns always looked so much better than everybody else's. He told me he wipes them down with Clenzoil to really bring out the best the finish has to offer.
99 out of 100 times, the guy sells out of everything he displays, and usually gets top dollar for his stuff.