Renault says Red Bull's decision to use a different fuel and lubricants supplier is likely a factor in why it's not getting the same power and performance from its TAG Heuer-branded power units.
The Renault factory team uses BP/Castrol for its oil and fuel requirements, as does customer team McLaren. However, this season Red Bull has opted to be supplied by ExxonMobil.
"They elected themselves for strategic and commercial reasons to go for their own partner, which we appreciate, and we support by homologating their product," said Renault Sport F1 managing director Cyril Abiteboul.
"I'm sure Exxon has to ability and capacity to catch up," he added diplomatically. "But they are a bit behind. The installation is also a little bit different.
"That's why there are small discrepancies, and I accept that."
Red Bull are the only engine customer team on the grid this year to differ in its selection from their manufacturer's works team.
Red Bull boss Christian Horner has previously defended the decision as an advantage. But Mercedes boss Toto Wolff criticised the move, saying that each power unit was optimised for specific lubricants and that Red Bull was missing out.
Mercedes itself uses Petronas products, as do its customer teams Force India and Williams. Similarly, Ferrari stipulates that Haas and Sauber should use Shell. Abiteboul agreed with Wolff that there was an inevitable performance hit with opting for a different supplier, however minor.
"We have more to come, and they could have more to come from the fuel," he said. "That's something that's clear.
"We can guarantee an equity of performance of what is in our control, which is hardware, which is software," he added. "Not anything other than that, like fuel."
Renault and Red Bull are currently locked in a tense stand-off about engine supplies for next year.
Renault wants a decision from the team about whether it will continue to buy power units from them in 2019. Red Bull is currently evaluating the engines Honda supplies to the junior Toro Rosso squad with a view to switching at the end of the season.
Both manufacturers introduced upgrades at the most recent Grand Prix in Montreal, and showed steps forward.
"We see from top speed we've been very competitive, we also see a bit of a step from Honda," agreed Abiteboul.
"We know exactly what [our engines] are providing and they are providing what we advertised they would a few weeks, if not a few months ago."
A mid-race technical failure for Fernando McLaren's McLaren had given Abiteboul a moment of anxiety about the reliability of the new units, the Frenchman admitted.
"I stopped breathing for a moment when I saw Alonso complaining about his ERS," he said. "But I understand it was another problem."
McLaren's racing director Eric Boullier confirmed that Alonso's retirement had not been related to the Renault engines.
"[Fernando] was forced to retire the car after losing boost pressure thanks to a broken charge air cooler pipe," Boullier reported. "Although very frustrating, it’s an unusual problem, and one that we don’t expect to affect us going forward."