In Pixar's Cars, Rust-eze is the sponsor on Lightning McQueen's first Piston Cup team.
Rust-eze is a fictional product company that sells a rusty-car treatment. Its owners, Rusty and Dusty Rust-eze, recruit Lightning after noticing his racing potential. The sponsorship gives the rookie his familiar red-and-yellow branding and places him in the Piston Cup competition.
Lightning initially treats the sponsor and its owners dismissively because he wants the prestigious Dinoco sponsorship. His time in Radiator Springs changes that attitude, and Rust-eze remains tied to his career even as the Dinoco storyline develops. The brand is therefore both a racing sponsor and part of Lightning's character arc.
Dinoco is a separate fictional company and a major reward Lightning hopes to earn. Nitroade and Mood Springs are also Piston Cup sponsors in the film, but they are not Lightning's first team sponsor. That distinction makes Rust-eze the specific answer.