Together, they won the 2014 and 2021 Bathurst 1000s, and 21 races in total through stints at DJR, two at Tickford Racing, and most recently at WAU, before De Borre chose to step back from the grid of top tier Supercars competition.
Mostert now has a new voice on his radio in the form of another former Tickford figure, Sam Scaffidi, who was previously James Courtney's Race Engineer.
The relationship is not entirely new, given Scaffidi was a Data Engineer for the Queenslander at the Campbellfield squad, but last week's test day at Winton was something of a milestone for the pairing.
“Obviously, I haven't worked with him for quite a few years, but he's really come on quite different to the guy I did know,” Mostert told Speedcafe following that test day in his Mobil 1 Optus Mustang.
“When I worked with him when he was a data engineer, he was really great from the driving point of view.
“Obviously, it's awesome how anyone who comes through engineering in Supercars, if they do that side of things first; help the drivers, learn what's going on there, and then obviously move into trying to make the car fast.
“I really enjoyed the day, I really enjoyed working with him.
“The way he's explained stuff is still like the same way I'm used to, which is handy – he talks the same lingo – so time will tell, we'll see how it goes.
“Who knows where everyone else ran [during the test day] but, overall, I was pretty happy.”
WAU co-owner Ryan Walkinshaw had already said that he sees benefits in a change for his team's spearhead, despite the longevity of the Mostert-De Borre partnership.
They were put together for the final two events of the 2023 season as a head start for this year, meaning last week's outing was their first test day as a drive-engineer combo.
Asked if their relationship has progressed as a result, Randle told Speedcafe, “Yeah, I think so.
“I mean, the first time we were together was Friday practice at Gold Coast, and a race weekend is a different thing to a test day.
“Your test day is very condensed, you do a hundred laps in a test day, you do 300km, and you're trying all sorts of different things, but it seemed to work well.”
Both Mostert and Randle are in action in this weekend's Bathurst 12 Hour, in a McLaren GT4 and a Mercedes-AMG GT3 respectively, before the Supercars season starts on the following weekend at Mount Panorama.