About Red Line
Red Line Synthetic Oil was founded in 1979 by Tim Kerrigan and Peter Filice in Novato, California, building lubricants specifically for the racing industry before expanding into the mainstream automotive aftermarket. The company has operated its manufacturing facility in Benicia, California since 1996, and in 1986 brought on chemist Roy Howell, formerly of Lubrizol, as Chief Chemist to develop its ester-based formulation work. Red Line built its reputation on polyol ester (Group V) base stock chemistry rather than the mineral or Group III base oils most competitors use, and the lineup now spans motor oils, gear oils, ATFs, assembly lubes, fuel additives, and its well-known WaterWetter cooling additive across automotive, motorcycle, marine, and industrial applications. Phillips 66 acquired Red Line's parent, Spectrum Corporation, in 2014, and Red Line now operates as a premier lubricants brand under Phillips 66.
The brand has a long motorsports résumé, including a role as official oil supplier for the Formula Mazda Challenge since 2009 and contingency and sponsorship programs spanning drag racing, road racing, time attack, and motorcycle racing. On RevRated, Red Line's manual transmission fluids are the most consistent name we see across gearbox threads for Miata, BRZ/86, Mustang, and VW/Audi/Jeep platforms -- the MT-90 is our top pick in that roundup, and the lighter MTL is our pick for cold-climate and Honda/Acura applications. Red Line's SI-1 fuel system cleaner is also reviewed in our fuel injector cleaner roundup for its PEA/PIBA dual-detergent chemistry.
Brand website: redlineoil.com
What Red Line makes
Red Line's range runs wider than the products we review. The lines we know best from our own testing are marked with a star:
Manual transmission & gear oils
GL-4/GL-5 ester gear oils and synchromesh-safe MT fluids -- featured on RevRated — featured on RevRated
Fuel system cleaners & additives
PEA/PIBA injector cleaners, fuel treatments, and fuel system additives -- featured on RevRated — featured on RevRated
Motor oils
full synthetic ester-based engine oils for street and racing use
Automatic transmission fluids
synthetic ATFs and D6/D4 spec fluids
WaterWetter cooling additives
surfactant-based coolant additive for improved heat transfer
Assembly lubes & greases
engine build and chassis assembly lubricants
Why we recognized Red Line
The RevRated Top Brand recognition isn't paid for and can't be applied for. We award it to the brands whose products consistently stand out in our hands-on category reviews. Red Line earned it on the strength of the products below, evaluated the same way every product on this site is: bought, run on real vehicles, and judged on merit.
Red Line products we rated highly
Red Line 50304 MT-90 75W-90 GL-4 Manual Transmission and Transaxle Lubricant - 1 Quart
Carries the deepest review record of any manual transmission fluid we found on Amazon, at 5,093 ratings and a 4.9-star average, and its straight GL-4 rating (no hypoid extreme-pressure package) means the brass and bronze synchronizer rings inside a synchronized gearbox aren't chemically attacked the way a GL-5 axle oil attacks them. It cross-applies across an unusually wide fitment list, including Mazda Miata, Subaru BRZ and Toyota 86, Ford T-5 and T-45 Mustang boxes, Dodge and GM NV3500, and a long run of VW, Audi, and Jeep gearboxes, and owners consistently report it fixes notchy, baulky cold shifting versus the factory fill.
Read the 10 Best Manual Transmission Fluids of 2026 guide →
Red Line 50204 MTL 75W-80 GL-4 Manual Transmission and Transaxle Lubricant - 1 Quart
A lighter 75W-80 grade of the same GL-4, yellow-metal-safe ester chemistry as the MT-90, which flows to the synchros faster on a cold morning and is the community-standard fix for the cold second-gear grind that Honda and Acura boxes are known for. It's popular well beyond Honda, with a following in S2000, Civic Si, RSX, and TSX communities plus VW, Audi, and BMW boxes that call for a lighter GL-4, and its 1,767 ratings at 4.8 stars is a substantial record for a grade this specific.
Read the 10 Best Manual Transmission Fluids of 2026 guide →
Red Line SI-1 Complete Fuel System Cleaner (2-Pack, 15 oz each)
A high-concentration PEA and PIBA dual-detergent formula that addresses both the baked-on carbon PEA dissolves and the soft varnish deposits PIBA removes, with cleaning efficiency reported at near-100 percent in controlled flow-bench testing. It has a strong following among track-day and modified-engine owners, where injector spray pattern consistency directly affects power output, and carries a corrosion inhibitor package that protects aluminum fuel rails, injector bodies, and throttle body housings from oxidation. At 8,657 ratings and a 4.7-star average, it's one of the more heavily reviewed fuel system cleaners in our roundup.
Read the 8 Best Fuel Injector Cleaners of 2026 guide →What earned Red Line the recognition
- The Red Line MT-90 is our top pick among the ten manual transmission fluids we reviewed, with the deepest Amazon review record in the category at over 5,000 ratings and a 4.9-star average.
- Red Line holds two spots in our manual transmission fluid roundup -- the MT-90 for most gearboxes and the lighter MTL for cold climates and Honda/Acura applications -- both built on the same GL-4, yellow-metal-safe ester chemistry.
- Founded in 1979 in Novato, California, and manufacturing from its Benicia, California facility since 1996, Red Line built its lubricants on polyol ester (Group V) base stock chemistry rather than the mineral or Group III oils most competitors use.
- An official oil supplier for the Formula Mazda Challenge since 2009, with contingency and sponsorship programs across drag racing, road racing, time attack, and motorcycle racing.
- The SI-1 fuel system cleaner in our fuel injector cleaner roundup uses a PEA and PIBA dual-detergent formula reported at near-100 percent cleaning efficiency in controlled flow-bench testing.