When an over-enriched combustible mixture enters the cylinders, the engine will rarely or never give full power, and you will drive with an overcooled engine (city driving), there will be no complete combustion of the combustible mixture, and as a result, oil carbon deposits and other products will form. The unburned fuel itself and condensate lead to the formation of slag (sediment) and acid. Unburned fuel is deposited on the cold cylinder walls and gets into the engine crankcase, and the oil film on the cylinder and piston is washed away. This has the effect of reducing the lubrication of the working surface of the piston and dilution of the oil, which worsens the lubricating properties depending on the fuel content.
Since some of the fuel dissolves in the oil during start-up, especially in winter (many starts with a cold engine - a high percentage of fuel in the oil), if the oil is significantly diluted, it must be replaced ahead of schedule.
Depending on the temperature, any oil has a tendency to decrease viscosity; with increasing heat, it becomes low-viscosity, which results in worsening adhesive ability and the compressive strength of the lubricating film. In the cold, the oil becomes thick and viscous, with fluidity decreasing and internal friction resistance increasing. This property is necessary for the use of motor oil, the fluidity of which would change little at variable temperatures.
When the engine is cold, it is necessary that the oil be of sufficiently low viscosity so as not to impede the operation of the starter and so that, as quickly as possible, from the moment the engine is started, the oil reaches all lubrication points.