Motor oil Vs. Tranny Fluid ... .Difference in results who knows? I used tranny fluid the last i tried. I used a 1861 Colt Navy "clone" and removed Italian proofs and "black powder only" markings. My barrel came out a plumbish color.If you look at it in the light you can see how purple it looks. I settled for that and was happy with the results. Its all in the prep work and cleaning and making sure nothing is contaminted if you want good results.I'm still wondering what oil to use. I have heard "dirty" motor oil, but I"m not going to do that - not to a high-gloss prepped gun. I have also heard that tranny fluid works. Dunno.