Mercedes boss Toto Wolff believes that rival team Red Bull have made a mistake by opting to go its own way with its selection of fuel and oil supplier.
Renault and McLaren both use BP/Castrol, but Red Bull have signed up with ExxonMobil instead. They're the only customer team in 2018 to differ from their engine provider's works team.
"Strategically you need to make sure that you are on the same performance levels," he explained. "And therefore have the same fluids as the works team."
Mercedes is quite insistent about Williams and Force India also using Petronas in their cars. Similarly, Ferrari stipulates that Haas and Sauber should use Shell.
Wolff explained that the power unit package was optimised for use with a particular fuel and oil.
By going a different way, Red Bull has to pay extra to conduct dyno running when there is a new specification from ExxonMobil to sign off.
McLaren was signed up with ExxonMobil in 2014 when it used Mercedes engines prior to its ill-fated switch to Honda. The common consensus is that it cost the team in on-track performance.
Wolff stressed he had nothing against ExxonMobil or any other oil and fuel supplier, and that it was just down to consistency.
"If you have a different spec of fuel, you need a completely different engine calibration," Wolff told Motorsport.com.
"You're running different calibrations on track, which doesn't give you any learning," he continued. "You're making your life complicated if you're having different specs. It's a philosophy."
Wolff felt that Red Bull had made a "strategic mistake" going for the cost-cutting commercial deal. He said that overall any cost savings would be swallowed up by extra dyno testing.
"The net number will be much lower than the value of the overall commercial deal," he insisted. "The relationship between a works team and a works team supplier of fuel and oil is always going to be more intense."