I'm trying to clean and old stainless steel pan which has burnt grease inside and outside. Ideally, the method used should be functional to clean the outer edges of the pan, therefore something a bit stick would be better. How can I do that without scrubbing or using commercial products?
You can often remove crud with electrolysis or lye (drain cleaner, or old fashioned caustic cleaner). I've read about people using a paste made from salt in alcohol or a bit of water for its abrasive qualities.
Electrolysis is accomplished by putting the pan in an electrolyte bath (usually baking soda or washing soda, but I would suggest lye for a dirty pan (wear gloves and goggles!)), and connecting the pan to the negative side of a DC a power source. The other end will be connected to a "sacrificial anode", which is usually a hunk of carbon or non-stainless steel. The current should be a few amps at most. 12-20 V power supplies work well, if they can supply enough power. Take care not to let the anode touch your pot. The pot regions closest to the anode (and in line of sight within the electrolyte bath) will get the cleanest. Let it work for a couple hours, adjusting the position of the anode periodically. If the reaction stops, take the anode out and scrub it. When you are done, shut off power, remove the pan, and scrub it. Electrolysis isn't magic, and the result won't be perfect.
Note that this is a description from memory. Long tutorials and videos have been published on this topic, and I urge you to seek them out before trying it.
You specifically don't want to hear about commercial products, so let me just mention that there are better and cheaper ways to clean your pan. But that would be a different question. :)
Edit: I see you don't want to scrub it, so the only option is to soak in caustic for 24 hours. It won't get all the crud off.
Try putting the pan on a burner and heating it up. Once its hot, some of the grease may liquefy, so the burned on grease may become easier to remove. You can then poke at it with a non-scratchy (plastic) spatula, and/or use a wadded-up paper towel. If you use the paper towel be careful not to burn your fingers!