I believe the main complaint with Traxxas fuel is the oil content. I found this quote on the internet.
"Some users feel that Traxxas fuel has a higher oil content than necessary, around 16-18%, which they believe is excessive. This high oil content can lead to a hotter burn and may make the...
Some unburnt oil will end up in the exhaust pipe and that is normal but if you end up with excessive amounts then my guess is, you're running to rich. 16% oil is too much imho. You want something with 8-12%. Id suggest O'Donnell's, Bones Brew, or even the regular Sidewinder with 12% oil.