You could use tags or notes for this. If you use a specific tag for each type you can view them separately in your fuel log. For example, here are fuel-ups of mine
tagged chevron.
If want to view the fuel-ups separately in your log book, you can use a note and then use the search feature to bring up matching words. So a single note with 'alcohol' or 'gasoline' would bring up those fuel-ups when you type those words in the search box above the Fuel Log. It wouldn't be much different from what you're doing now, maybe just a little more descriptive.
Fuelly doesn't do any sort of separate analysis based on tags or notes. For that you might need to go to the
export feature and write your own reports in Excel. That way you could graph the two types of fuel separately on your own. But notes or tags might still be helpful in Excel so it's easy to separate the two.