Red Line Oil Synthetic D4 Automatic Transmission Fluid (1) RED LINE D4 ATF - Synthetic Dexron III and Mercon fluid meeting the requirements of most transmission manufacturers, while satisfying GL-4 gear oil requirements. Superior stability allows high-temperature operation without varnishing valves and clutches. Also provides tremendously improved shifting in cold weather. The best low-temperature shiftability for manual transmissions and transaxles which require ATFs.