Engine Oil

V-Twin Transmission Tech: A Better Idea Than Using Motor Oil in Your Transmission

Lubricants are a hot topic in the V-Twin community. Many people ask the simple question: “Is it best to follow the dealer and put motor oil in my primary case, transmission and engine? If I could find the right product for each application?” The engine, transmission and clutch of most American V-Twins have three distinct lubrication systems.

Experts and knowledgeable enthusiasts will tell you to not use the same motor oil in each of these compartments. The dealership may manage its inventory with one type of oil from the factory, but that doesn’t mean this product is the best for you. Motorcyclists who want to preserve their investment and get the most out of their bikes will find it more practical to use a synthetic 20W50 motor oil.

Is it wrong to use motor oil in the transmission? Motor oils have additives that keep contaminants suspended. Transmissions don’t need to deal with byproducts from an internal combustion engine. So why not choose a gear oil that’s optimized for those conditions? This is also true for the primary case. The problem with compromises is that they can’t do all the jobs well.

Red Line is proud to be able to offer motor oils. However, Red Line’s “ShockProof”, a unique product used in V-Twin transmissions, is truly revolutionary. Red Line’s Heavy ShockProof(r), Gear Oil, was first used by many Red Line enthusiasts. This same product is also available in V-Twin Transmission Oil. It’s also the same fluid used in the differentials of top-of-the-line Top Fuel Dragsters around the world.

This pink gear oil acts like liquid grease. It contains suspension solid microscopic particles that act as an extreme pressure agent. The unique solid dispersion cushions the teeth and prevents them from becoming worn. It is classified as a 90W oil, so it isn’t very heavy. ShockProof oil gear oils also stick extremely well to metal, which is a major part of their noise dampening and wear control. Although the early V-Twin stock gearboxes were not known for their quiet operation or smooth shifting, they are still very popular. After using V-Twin Transmission oil, you will no longer feel the shifting clunk at stoplights. This is confirmed by the large, unrequested comments on V-Twin’s most popular V/Twin sites.

The oil can’t prevent all things. Even if your bike is newer, lugging it around can cause noise and wear to the transmission. For example, ShockProof oil can reduce noise and improve gear life in the 2007/09 H-D(r), Cruise Drive 6-speed transmissions. However, its straight-cut fifth gear is very noisy. Many consider the motor at 96 inches too powerful, especially when it is matched with the 2.79:1 final drive ratio. The bike can lug in 6th but makes a lot of noise in 5th. V-Twin Trans Fluid helps but does not stop the bike from lagging. It’s not necessary to reach 90 mph in 6th gear to avoid lugging. However, it is certainly fun!

Aftermarket experts point out that the new bikes have a 103-inch motor and a 2.87:1 final drive ratio. Harley has addressed some of these issues with their latest version, and techs who have taken apart the bikes say that it’s a great transmission.

Experts have found that the main drive bearing races in H-D 5-speed transmissions between 1980 and 2006 had wear issues. They were also machined with large clearances, combined with its heavy shifter drum to create a noisy shift. Red Line’s V Twin Transmission Oil with ShockProof can help to reduce noise, wear and shifting problems. The late ’90s boxes have a more modern design due to the hollow shift drum. Our gear oil can still improve these boxes. Our ShockProof fluids have been used greatly in aftermarket transmissions for Evos such as Jim’s, Baker and many others. Many of these companies are our customers.

ShockProof is only for wet-sump applications. You don’t want it to be run through coolers, filters, pumps or other equipment. Imagine how it would stick to the inside a small hose with low-pressure pump pressure if attracted to your gears. You should also keep in mind that the V-Twin Heavy ShockProof/Viscosity (90W) is the one you want to use. We have not seen the same benefits when using our 80W Motorcycle Gear Oil, Lightweight, Superlight, or ShockProof(r) examples.


Red Line has been riding since before it was fashionable (Tim Kerrigan Jr., an AHDRA Pro Fuel Champion), but we were quick to jump on the Buell(r), dating back to the late ’90s. We love the Sportster(r), American-style engines and road racing style. Technically, transmission and engine tips aren’t specifically targeted to this community. We will, however, try to share some of our knowledge with them.

Our friends also enjoy these bikes at Hal’s Speed Shop, New Berlin, Wisconsin (www.halspa.com). They have extensive experience in modifying these bikes for drag racing, road racing and street riding. Red Line has been very fortunate to have them as a technical resource over the years. They also back up our amazing success with Lightweight ShockProof (r), closely mimics the 80W Motorcycle Gear Oil with ShockProof (r).

Rob Schopf, Hal’s, reminds us that they prepared many 5-speeds for road racing in Buells and 883s. They noticed accelerated wear to the main drive gear. This was eliminated by Lightweight ShockProof(r). Rob states, “If the fluid fixes this on the race bikes it should keep street-ridden bikes running for quite a while.” H-D made the design more efficient using a fine-tooth/ high-contact gear patch on the later bikes instead of the wider, older teeth.

Hal reminds us that the greatest performance gains come from removing drag and resistance. ShockProof oil blends have the lowest coefficients of friction and are more popular than any other on the market. This is just one of many steps to ensure your bike runs at its peak.

The gearboxes of the latest Sportster models (2006-2009) are significantly improved. They no longer produce oil from the breathers. They should be as it is difficult to check the transmission wear on later bikes. The “trap door” that you used to access the transmission is gone, so you will need to separate the cases to view inside. Red Line recommends not running V-Twin Transmission Oil or Heavy Shockproof in a Sportster/Buell Transmission. Red Line does not recommend running V-Twin Transmission Oil or Heavy Shockproof in a Sportster/Buell transmission. It is important to always warm up your bike before draining the oil or changing the gear oil. This ensures that the transmission gets the best possible flush.


Red Line’s MTL (r) gear oil makes it easier to get an American V-Twin’s primary case lubricated. MTL has the balanced friction required to make a synchronized car shift. This lack of slipperiness makes MTL a great match for a V-Twin’s wet clutch.

This fluid is now packaged as V-Twin Primary Case Oil. The fluid’s balance friction and light viscosity provide excellent clutch operation for low power losses. It is the 70W80-GL-4 gear oil. However, it has stable ester base stocks, which ensure consistent film strength across a wide temperature range.

Simply put, don’t transport your bike if the primary chain or clutch needs to be maintained. The 96-inch engines were equipped with an automatic, spring-loaded chain tensioner in 2007. This design, when lugged, can cause the bike’s high gear ratio to ratchet its chain too tight, which can lead to heat-soaked parts. We recommend that you switch to our Primary Case fluid.

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