From the Rotella website:
Can heavy-duty diesel oil be used in motorcycles?
Motorcycle gasoline engines may not seem in the same league as the big displacement diesel engine under your hood, but they share some of the same lubrication requirements. So yes, in many cases, a premium heavy-duty universal oil capable of serving both diesel and gasoline engines is the best choice for your bike.
The high power-to-displacement ratio of a motorcycle engine means rod and main bearings are subjected to loads that are not normally found in passenger car engines. The valve train is also highly loaded, and requires extreme pressure boundary lubrication. The same can be said about gears in the transmission, which are normally lubricated by engine oil. Oil additives containing phosphorus protect these highly loaded extreme pressure areas (in both gasoline and diesel engines). Because diesel engines have higher loading of components, more of the phosphorus-containing additive is present than in typical passenger car oils. And with advanced catalyst systems for gasoline engines, the phosphorus content has been declining in passenger car oils.
Since many bike engines are air-cooled, and tend to be operated at high power outputs and speeds, their lubricating oil needs to be more resistant to high temperature oxidation. That’s another advantage of a premium universal oil. Another thing you want in your motorcycle is oil that has excellent viscosity control, so that with use it retains high temperature viscosity. Some multiviscosity grade passenger car oils, subjected to extreme loads, can quickly thin out. Their viscosity can drop to the next lower grade.
One last thing to consider is whether oil contains friction modifier additives. For improved fuel economy, most passenger car oils have such an additive. But the wet clutch in your bike doesn’t perform right with friction modifiers. Universal engine oils don’t have friction modifiers.
Be careful choosing diesel oils. Not all of them are universal. In addition to the API Service Category CI-4 PLUS for diesels, look for API Service Category SL.
Premium universal oils like Shell ROTELLA® T Multigrade are formulated for heavy-duty performance, and your bike engine has some heavy-duty challenges for oil. For optimum performance, be sure your oil is up to the challenge.