Team mugen went through 6 different fuel manufactuers and decided that byrons was the only one that had world wide distribution capabilitys so that anyone would be able to use the same fuel that is used by the team....
Not to say that every team driver uses this fuel, some have sponsorships from other well known fuel makers....
No team driver is forced to run mugen fuel....
If you notice in the 2001 wolrds the fuel was byrons not mugen
Granted they are the same fuel but at that time they're was no partnership yet....