Viscosity is the single most important property of a lubricant. When I am working as a consultant and designing a lubrication program, one of the first steps I take is to calculate required viscosities. Getting the right viscosity is of the utmost importance. The selected viscosity needs to be pumpable at the lowest start-up temperature while still protecting the components at in-service temperatures.
Typically, diesel engine oil will have a higher viscosity. If we were to put this higher viscosity in a gasoline engine, several problems might arise. The first is heat generation from internal fluid friction. I’ve covered before how this heat affects the life of an oil. A good rule of thumb is that for every 10 degrees C you increase the temperature, you cut the life in half.
The second problem is the low-temperature pumpability of this higher viscosity. During cold starts, the oil may be very thick and difficult for the oil pump to deliver to the vital engine components in the lifter valley. This most certainly will lead to premature wear, as the components will be interacting without the benefit of lubrication.