Ferrari and Mercedes were the subject of a detailed investigation after the Canadian Grand Prix to ensure neither outfit was using trick oil systems to boost performance, Motorsport.com has learned.
With the two leading the championship, there have been suggestions that they could be exploiting loopholes in the regulations to try to gain a performance advantage.
Such fears were the catalyst for an FIA clampdown on the fuel systems in the Spanish Grand Prix as the governing body moved to stop teams trying to store petrol beyond the flow meters to try to get a power boost.
Now, the focus has shifted to oil systems and whether or not some teams have been adding performance-boosting chemicals to the auxiliary oil tank so that they can be used in the engine to help add horsepower.
The FIA confirmed that following free practice and qualifying in the Canadian Grand Prix, oil samples from Lewis Hamilton's and Sebastian Vettel's cars were taken away for detailed forensic analysis.
A report issued by the FIA afterwards said: "The viscosity analysis showed a slight reduction in the viscosity of the oil in the main tank compared to that of the equivalent auxiliary tank in both cars and the differences can be attributed to fuel dilution, as expected under normal operating conditions.
"The InfraRed analysis showed no significant difference between the oil in the main tank and the one in the equivalent auxiliary tank in both cars.
"Further the GC analysis showed no differences between the oil in the auxiliary tank and the one in the main tank for both cars. It also showed no difference between the oil in the main tank on Friday FP2 and Qualifying for both cars.
"Headspace analysis at 120°C confirmed the presence of fuel in the main tank oil samples (both FP2 and Qualifying), whereas none was detected in the auxiliary tank oil samples.
"This, again, is due to the expected fuel dilution of the used lube oils under normal operating conditions"
The FIA made it clear that following the analysis, it was satisfied that neither team had been in breach of the rules.
"From these results, it can be concluded that the auxiliary oil tank is not being used to add performance-boosting components, either to the main oil tank or to the combustion chamber via the sump breather into the air intake," the report added.
The oil system investigation shows the lengths that the FIA is going to to ensure that all teams comply with the regulations, and that it addresses concerns rivals have about their competitors.