This must be done in one playthrough. You cannot just reload and use the different combinations. There are a total of 4 items to mix with chem fluids. This means 8 total combinations. Chem Fluid is yellow, while Strong Chem Fluid is red.

There is an easy way to keep track of this. While in your inventory, press RB to look at Combined tab. It will show you which ones you have unlocked. The top row is First Aid Med, Handgun Ammo, Burner Fuel, and Psychostimulants. The second row is Strong First Aid Med, Enhanced Handgun Ammo, Flame Rounds, and Neuro Rounds. Read below to see how they are all crafted.

Combine Chem Fluid with
- Herb = First Aid
- Gunpowder = Handgun bullets (10)
- Supplements = Pyschostimulants
- Solid Fuel = Burner Fuel (100)

Combine Strong Chem Fluid with
- Herb = Strong First Aid
- Gunpowder = Enhanced Handgun bullets (10)
- Supplements = Neuro Rounds (2)
- Solid Fuel = Flame Rounds (2)