Headlight Fluid is a running gag that is first mentioned in The Rookies when Simmons and Grif decide to send Donut on a fool's errand, by asking him to pick up "elbow grease and headlight fluid". This eventually causes all later events in the series to happen and "headlight fluid" has been mentioned numerous times in future seasons and sagas. While the gag usually involves Donut, it has been involved with several other characters.
While it became popular after it was used in Red vs. Blue, headlight fluid doesn't originate from the series. It was invented during a prank call made by Detroit’s Channel 955 radio DJ Spike which was originally featured as a segment on the AM radio show Mojo in the Morning and subsequently released on a compilation CD in May 2001. Titled “Clueless Auto Repair,” the call consisted of a person posing as a mechanic advising a woman on the state of her automobile repairs. At some point, Rooster Teeth learned of the prank call and applied the idea to the web series.
- "Headlight Fluid" is the name of Rooster Teeth Animated Adventures #4 where Gavin Free tells a story of his work with a professional stunt driver who had an accident and says headlight fluid when he meant windshield washer fluid.
- In Halo 4's Spartan Ops there is an easter egg that has Caboose say through the radio, "Six crates of headlight fluid and elbow grease inbound now."
- In the series, it seems that headlight fluid is real, as some characters suggest getting some for equipment and there's advertisement of Stupid Brit brand headlight fluid on Basebook in Reconciliation.
- There are some merchandise from Rooster Teeth that referencing the running gag, some using an image from Animated Adventures #4.