@Silverfin--I'm not much of a chemist. When I took a chemistry course, the only elements were earth, air, fire and water. However, isn't nitric oxide one of the byproducts of combustion and isn't it possible that this could chemically change to NH3? If Shell has a nitrogen additive, might this not increase the presence of nitric oxide?
As I said, my knowledge of chemistry is pretty rusty, but you are a chemist so you would know if this is possible. I have a 2003 Toyota 4Runner V-6 which is probably the same engine that is in your 2007. I haven't noticed an ammonia smell, but I rarely use Shell gasoline.