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"Last winter at our small construction company in Minneapolis, we attempted to start the engine of a dozer that had been sitting outside for several days. Since last running the engine, outside temperatures had slowly dropped to around minus 13 degrees F (minus 25 degrees C). The dozer's motor oil viscosity is SAE 10W-30. The engine cranked slowly but still started. However, the oil pressure was very low and stayed low. What might have caused this? What risks or damage to the engine can result?"
The high viscosity of the engine oil did not allow oil pressure to build quickly and may also have caused the system to go into bypass, thus the low oil pressure. Severe damage can result to the engine from lack of lubrication, including seizure of main and connecting rod bearings, piston scuffing/seizure, etc.
One solution to the problem would be better storage of the machine exposed to these severe low temperatures. Covering the engine with insulating material or raising the entire machine temperature with space heaters within a tent-type cover before attempting to start the engine might be an option.
If starting in such low temperatures is expected, using a lower viscosity oil like an SAE 5W-30 may help. The oil should be replaced before the winter season, not at the time the cold-starting is attempted.
Cold-starting should not be attempted unless the oil will "flow" off the engine dipstick. This is a quick, simple visual test. You also need to determine if the in-service lubricant has the ability to provide proper cold-start performance at this starting temperature.
Lube oil tests that show the viscosity at 40 degrees C and 100 degrees C would indicate that oil has degraded from new. Soot loading would be high. Oxidation and sulfur would also read high.
In this unit (diesel engine), soot loading would be the primary factor for the increase in viscosity. Soot loading also increases the oxidation rate of the oil, which in turn increases the viscosity.
Sulfur in the oil (from fuel) could also be high, either from extended oil change intervals or from the engine running too cold (cooling system problem or short operating cycles).
Condensation (and the rise in acid formation) would also be a factor in a cold-running engine. All of these combined factors would lead to a severely degraded oil with high viscosity in a short period of time.