I don't know exactly why it changes smell, but I think that some elements evaporate, leaving the turpentine which does not evaporate as easily.
I expect that Gumout makes a product that is the same as B12. You should be able to get B12 at any auto parts store. It is mostly benzine.
If the gas has been in there 10 to 15 years, you may have to clean the tank once the gas is burned or drained out.
Best of luck,