This is not a well kept secret. There is a car care product available everywhere called "Nu-finish." I get mine at Walmart. Add an ounce or so to your tumbler whenever you clean your brass and it comes out shiny and slick, just like new. It is an inexpensive and readily available alternative to the formulas the gun companies sell. It works great with either walnut or corn cob media. People often ask me which I prefer. I can't tell the difference. They both work well.
People also ask how long to tumble their brass. That is a subject left to much debate. I know people who tumble theirs as little as 10 minutes and as long as 8 hours. Generally I tumble mine for about 2 hours. Maybe that's too long, but it shines like new and it's not like I have to sit and watch it.
I've never really tumbled loaded rounds, but that's because I never felt the need to. It would make your rounds more cosmetically pleasing and it's possible that the cleaner shinier rounds would go through the mags and chamber better. From what I understand, and believe there is no danger or downside to tumbling loaded rounds. But like I said I've never done it and have no personal experience with it.
While I'm on the subject I should mention that the Ike's pheasant committee still has corn cob tumbling media for sale. $10 for a 10# bag. That's actually a pretty good price. I was in Gander Mountain Thursday and they were selling 6# for $16.99+tax. You can pick some up at any of our matches or set-ups.