The 22-year-old moved to FPR this year after racing with DJR for the majority of the 2013 season.
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“I'm basically here to make sure their cars are like ours and everything is working as it should with a satellite team,” Mostert told Speedcafe.com.
“I just did a handful of laps to make sure the cars feel like mine, which gives them the confidence that their data is the same as ours.”
Mostert said he was impressed by just how similar the two cars, which run different engine packages, were.
“It's quite surprising, I raced with DJR last year and jumped over to FPR this year and it didn't feel too much different,” he said.
“They are two different teams that build and prep the cars so it's quite amazing that they're so close in feel.”
Mostert's appearance at the test followed a similar move last year, when FPR's Mark Winterbottom drove a DJR Falcon ahead of Mostert's debut for Johnson's squad.
DJR stressed last week that its relationship with FPR will be unaffected by the departure of its sporting director Campbell Little, who had previously sat in on FPR briefings during race meetings on behalf of the customer team.
Downplaying any advantage that FPR could gain from DJR's test ahead of the August 1-3 race meeting at the venue, Mostert won't enter the Ipswich event short of recent miles.
The former Australian Formula Ford champion completed FPR's own test day at Winton on Monday before undertaking a ride day with the team on Tuesday and heading to Queensland Race today.
“I'm a bit dizzy to be honest after three days of watching cars go around,” he said.
“Our test day went really well so we're looking forward to see if we've improved, not so much at Queensland Raceway, but more so at Eastern Creek when we get back to the hard tyre.
“That (hard tyre performance) was our main focus.”
“It's always nice to go back to a track that you've had a win at,” he said of Queensland Raceway.
“You have confidence in your baseline set-up, but in saying that a lot of things change in 12 months. You can't go in there with too high hopes.
“You've just got to get in and try your best and if you're lucky enough to repeat what you did the year before you'll know you did a good job.
“But it can be anyone's day at any time; the field is so close this year, even more so than last year.”