It seems like those evap codes can sometimes take forever before the CEL goes out. The last time I had one, and I was pretty sure I fixed the problem but the CEL would never go away, I disconnected the negative battery cable to get rid of it and it never came back. As far as a fuel additive to clear that code, I can't se how any fuel additive would make an evap code go away . . . aren't they usually initiated by a leak somewhere in the evap system?