Fuel blends are very heavily guarded and regulated. The rules also change as teams with clever boffins in dark rooms find ways of “entering into the spirit” of the FIA law. Regulations have also changed on fuel for 2018 (most notably diolefins that likely reduce combustion temperature and so be more predictable) These additives ARE however in the Oil the cars use.
If the Italian press is correct then it would appear that orders from the very top have emerged from "secret Maranello rooms" that the F1 team have removed chief PU designer Lorenzo Sassi from office despite the boost in performance he has created, with the gap in performance to Mercedes reduced or even non-existent.
Fernando Alonso's career path has been called into question since his return to McLaren, however, once again his manager, the Italian businessman Flavio Briatore (banned for fixing the Singapore Grand Prix whilst chief of Renault F1) has stirred the hornet's nest once more with his latest promotional statement: ‘Don’t exclude a Ferrari return for Alonso’