Odorless mineral spirits for me. Dry time only. I just mounted an eider to see if it was much of a difference? I must say that the bird took a considerably longer time to dry (of course it was an eider), but the skin remained what I'd call humid (if that's a good description? lol), along with the bill juncture. I actually preferred this route due to fact that the bird skin wasn't drying faster than I liked. It seemed to groom much better too? When I started grooming it with my soft polishing brush, the feathers aligned very nice. Maybe it was just one of those birds, bit it sure seemed different. I know I will be doing my next bird without the mineral spirits to see if it was just a fluke, or my new way to do this?
Gas is no substitute for good fleshing. If you clean the grease very well, and wash the bird and rinse very well, the mineral spirits (or gas) will do nothing more than aid in dry-time. Does gunk seem to come out after a good fleshing and washing? Yes it does. As I have gotten much better at the degreasing and cleaning process, I see far less gunk in the bottom of the gas/water solution than I ever did before. As a matter of fact, really I just see stained water for the most part. Their specific gravities are completely different, so the heavy water simply evacuates from the feathers and skin when you add the "gas", and settles to the bottom of the bath. The gas can be re-used. Simply pour the clean, clear liquid in a container, and use as many times as you can before it becomes too saturated with fat/water, and becomes cloudy.
Another great question that has only been asked 126 times in the past. lol