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Red Bull insists it did not keep Max Verstappen's Formula 1 engine upgrade secret from Daniel Ricciardo, after the Australian only found out about it when he was told by the media.

Verstappen was handed Renault's new-spec engine, which features 2018 development parts, for the United States Grand Prix weekend.

The changes were estimated to be worth up to two tenths of a second, and Ricciardo was caught by surprise after qualifying when told that his team-mate had a slight performance advantage.

Ricciardo's lack of knowledge about the engine situation prompted conspiracy theories about Red Bull favouring Verstappen, especially in light of the Dutchman's new long-term contract.

But the team says it never thought to mention the upgrade to Ricciardo because it did not believe there was a worthy performance gain from the new power unit.

"It is very difficult for us to measure what the difference is," said Red Bull boss Christian Horner.

"Renault have only made one engine available to each team and Max was due a change here.

"So obviously it made logical sense for that to go in his car this weekend and take the [grid] penalty.

"The intention with Daniel, and it is still the intention, is to not to have to take a penalty between now and the end of the season."

Pushed on whether being open with Ricciardo was an attempt to keep the different spec hidden, Horner said: "The spec is so marginal. We cannot measure the difference.

"It is within a fuel flow meter, so it is the same."

Ricciardo was forced out of the United States Grand Prix after his Renault engine shut down when it began losing oil.

Without that, Horner reckoned the Australian would have finished on the podium.

"Daniel was losing oil with the engine and then it went into a safe mode, so I hope that that engine is salvageable.

"It is a great shame because he was hustling Valtteri [Bottas] very hard and looked very competitive at the beginning of the race.

"I think he would have been on the podium quite straightforward."