Bear in mind that yep, the fuel system will circulate and mix the fuel/oil..........EVENTUALLY. But what about all the oil left in the filler tube? Its a corrogated tube (in sections) with lots of ribs to hold oil. When you're adding .5 liter's to a tank of fuel, and 1/3 of it stays in the filler hose is it ideal?
I add the oil first, always have in anything I premix fuel for. If you've done it the opposite way in a gas can, you'd see why.
Also, I use amsoil with TCW-3 but will use anything I come across with TCW-3 if I'm in need somewhere away from home.