I peeked at that stuff the other day. Never used it. I would run it for 10 gal or so and look at the piston on one of your stihls by pulling the muffler. If the skirt looks clean and the plug is not chunky than it's good. It's an fd oil, so the detergents are going to be way up there. The only diff between fb, fc, and fd is the detergent level.
I run the stihl super hp oil(it's and fd) for the first gal of the season, and the stihl ultra oil(full sen) for the rest of the year.
Give us the scoop after a few tanks.