mswaim is correct, there is no fuel additive that has a sulphurous smell. The culprit is likely your catalytic converter if the smell is indeed sulphurous.
People's perception of smells can be ambiguous. Are you certain the aroma is not similar to "airplane glue" ? If so, jackwells would be correct. It seems the glue smell is worse in hot weather - hence all the recent posts about it.
Here's a test you can do: Buy a CO (carbon monoxide) indicator. They're cheap. We pilots all have them. Try
www.sportys.com
If the CO levels are high, a reactive disk turns black, and you have proof positive to present your dealer. This will force them to react. If not, buy a tube of airplane glue and (carefully) compare the aroma.
Please keep us posted.