The oil-burn controversy came to light earlier in the year when Red Bull Racing said it suspected its rivals had found a way around the regulations surrounding the internal combustion component of the hybrid power unit.
In response, the FIA took a closer look at oil usage and consumption, and the chemical composition of the oils being used.
The FIA has now confirmed that the use of chemicals in oil that could help improve combustion is not permitted.
Head of the F1 technical department, Marcin Budkowski, wrote to the teams to re-emphasise the FIA’s position that the use of oil as fuel is prohibited.
"For the avoidance of doubt,” Budkowski wrote, “the only fuel that may be used for combustion is petrol, and the only permitted characteristics of that petrol are clearly set out in Article 19 of the technical regulations.
"Even though the technical regulations do not directly specify the...
Read more: FIA pursues F1 teams over illegal oil burning - Inside Sport