Keeping all of this information in mind, if the optimal oil operating temperature is not achieved, the engine will be in an inefficient running and lubrication state.
If the oil operating temperature in the sump is below 90°C when the water cooling system is at optimum temperature then there will be additional cooling of the oil cooled components such as the piston skirts and crank shaft. This will interfere with the expansion rate and extension size as well as operating clearances between these components. For example, the cylinder liner will expand due to combustion heat but the piston will then be cooled excessively due to the low oil temperature. The clearance between the piston rings, piston skirt and the cylinder liner will not be optimal, allowing for inefficient combustion which will, in turn, aggravate situations such as fuel dilution, soot generation and fuel consumption efficiencies.
An additional problem that is a lot less visible is the build up of acids in the oil. In a cold engine there is condensation from atmospheric moisture in the air, and water is a normal combustion by-product. As fuel is burned there are combustion by-products deposited in the oil and in the presence of water these combine, forming harmful acids. If the oil temperature is too low, it creates an ideal environment for this to take place. Under normal operating temperatures, most of the water evaporates and the acids cannot be formed so easily. In the low oil temperature situation they are created and can cause chemical acid attack of the soft metals, which can also decrease the life expectancy of the engine.
This problem will also be aggravated by the fact that at low temperatures the oil will be too thick to be filtered efficiently and therefore the filter bypass valve will be in the semi-open position, almost continuously allowing a percentage of unfiltered oil to move through the lubricated areas. This could quite easily lead to an accelerated wear situation. Due to over-cooling, shaft seals and sealing efficiencies will be compromised possibly causing oil leaks and/or contamination ingress. All of these factors can result in a reduced life expectancy of the engine. A safe oil temperature and viscosity range is illustrated in Figure 8.
