BMW may change third driver plans

The German car manufacturer were joined by Williams in being the only two teams to run a third driver in Friday's practice session at last weekend's Australian Grand Prix.

And with the situation obviously limiting the mileage and preparation for their race drivers, there have been suggestions that the team would be better off just focusing on Nick Heidfeld and Robert Kubica for both the Friday sessions.

Although Theissen has previously appeared set on running Vettel every Friday, he has now admitted that it is likely the decision will be taken on a race-by-race basis.

"We will continue like this, but I would not say necessarily for the entire season," he told autosport.com. "It depends on the situation on a race-by-race basis.

"For instance, if we had expected a dry session on Friday morning and a wet one in the afternoon then we would have given Robert the chance to do both sessions."

Theissen has admitted to being frustrated that F1's current testing restrictions make it very difficult for him to bring on and develop young drivers like Vettel outside of grand prix weekends. He also suggests Fridays of grands prix should be unrestricted test days, rather than extended practice sessions.

"I find it difficult to evaluate a young driver and give him the chance to grow in F1," added Theissen. "With five one-car tests during the season, what can you do?

"I was very much in favour of the original idea of having a test day on Friday. That would cure the problem in my view, you could give your young drivers enough time as well."

Renault boss Flavio Briatore has also suggested that Friday's current rules are too restrictive, having been unable to give Heikki Kovalainen any more running after he stopped early in second practice in Australia.

"We are not allowed to use the T-car," said Briatore. "When we had the rule (to change Fridays), in my mind it was going to be a testing day and in the end it is just a preparation day for the race. We did not put it together (right)."