A reader asks: A lot of motorcycle owners use Shell Rotella T6 diesel oil in their engines/transmissions. Is there any downside to using diesel motor oil in motorcycles? As Shell Rotella can be found fairly cheap, why would I want to use the more expensive AMSOIL Motorcycle Oils?
Our Answer: Diesel motor oils can provide (somewhat) adequate service in motorcycle engines and transmissions. But diesel oil used as a motorcycle oil won’t necessarily provide good value. Even if it has a lower price tag.
Dedicated motorcycle oils have a precise base oil and additive package combination. This balance of ingredients delivers specific protective and performance characteristics. This chemistry will not be found in diesel motor oil.
The reason that diesel engine oils can be used in motorcycle engines is that they do not contain friction modifiers. The friction modifiers found in passenger car motor oils (for gasoline-powered car/truck engines) will quickly glaze wet-clutch plates.
Here are some of the features of AMSOIL Synthetic Motorcycle Oil that are not a consideration in the design of diesel engine oils.
Motorcycle Oil Vs. Diesel Engine Oil In Transmissions
The biggest knock on diesel engine oils in this case is that they are not formulated for motorcycle transmission use. When used in common sumps (for both engine and transmission), diesel oil can be prone to sheering down and losing viscosity. Some diesel engines have timing gears, but this is nothing like the concentrated thrashing of a motorcycle transmission. The meshing of gears in motorcycle transmissions can rapidly sheer down oils that are not equipped for that type of use. Therefore, this rapid thinning will also hurt the diesel oil’s ability to protect the engine in common sumps.
AMSOIL Synthetic Motorcycle Oils are loaded with ingredients that withstand heavy transmission use. AMSOIL is also famous for lowering transmission friction for buttery smooth shifting. This factor alone is well worth the step up to AMSOIL.
Wet-Clutch Performance And Longevity
Diesel oils may not cause clutch plate glazing, but they are not formulated with wet-clutch performance and preservation in mind. AMSOIL Synthetic Motorcycle Oils are formulated to fight clutch fade and provide consistent clutch “feel”. They deliver steady clutch performance under high heat and extreme loads. Dedicated motorcycle oils will maintain the correct frictional properties over long service intervals.
Motorcycle Engine Oil Withstands High RPM and Heat
High revving motorcycle engines can red-line anywhere from 5500 rpm to well north of 10,000 rpm. They can also generate extreme heat. AMSOIL Synthetic Motorcycle Oils withstand extended periods of high RPM and extreme heat. The low-friction formula provides cooler operating temps. AMSOIL has performed extensive testing to prove that their motorcycle oils can be exposed to long periods of extremely high heat and high RPM without breaking down.
AMSOIL Adds Anti-Rust Protection To Motorcycle Oil
When designing their synthetic motorcycle oils, AMSOIL borrowed anti-rust and corrosion technology from their Marine Engine Oils. Engine internals are protected from moisture ingestion and long periods of off-season storage. Diesel engine oils would not contain this level of anti-corrosion ingredients and would not be as effective at protecting engine parts from rust.
Service Interval: Some Motorcycle Oils Are Extended Drain Capable
AMSOIL designs their V-Twin and Metric Motorcycle Oils for long service intervals. They can stay in service for up to double the interval as suggested by the engine manufacturer. AMSOIL formulates these oils to withstand the heat, transmission gear thrashing, high RPM. They also have the capacity to hold clutch, transmission and combustion deposits in suspension over this interval. Diesel engines oils are not designed to provide this length of service. As mentioned, the meshing of gears in transmissions will quickly sheer down diesel motor oil and reduce its service life.
As we’ve shown, the formula for motorcycle oil is different from the chemistry required for diesel engines. Diesel engine oils are missing several key components. As a result, this can compromise wear control, performance and engine oil life. AMSOIL Synthetic Motorcycle Oils provides superior performance, better engine and transmission protection and longer service intervals.
Diesel engine oils like Shell Rotella may be cheap, but that’s about it. A good synthetic motorcycle oil like AMSOIL is about the most cost-effective upgrade one can make to a motorcycle.