Post-season, he stepped down from IndyCar full-time to move to Penske's IMSA SportsCar Championship program. Castroneves was entered into both the IndyCar Grand Prix and the Indianapolis 500 as part of his 2018 campaign; he prepared for the races by sharing notes with his teammates and familiarizing himself with the car. Castroneves' team employed a strategy allowing him to finish sixth in the IndyCar Grand Prix. At the Indianapolis 500, he started from eighth place, before losing control of his car in turn four and crashing on the 146th lap for a 27th-place finish. In the 2019 season, he again entered both the IndyCar Grand Prix and the Indianapolis 500. Castroneves finished the IndyCar Grand Prix two laps behind the leaders in 21st. He started 12th in the Indianapolis 500, finishing the race 18th following a pit lane collision with the rear of James Davison's car earning him a drive-through penalty. He began the 2020 Indianapolis 500 from 27th and finished the race in 11th.