Why do many people report a lower oil pressure reading when their crankcase oil level is low?
We have to ignore the possibility of the pump displacing anything but oil in order to have a sound theoretical discussion. But it is possible that there is a large proportion of air trapped in the oil from frothing as it returns to the pan, or that the pickup tube — or its seal to the pump or block — has a pinhole in it that is covered when the oil level is higher (this should produce a relatively sudden change in pressure as the pinhole is uncovered, as opposed to a gradual change).
We also have to assume that the pump is not allowing oil to drain back out. But this is known to be false; an oil pump turned slowly enough will pump nothing but air. So there is not a linear correlation between pump speed and the volume of fluid pumped, because the faster the pump turns, the smaller the time period in each rotation where drainback could occur. The drainback effect will be more severe when the oil that wants to drain back is far above the level of the oil in the pan. So it would be expected that a worn oil pump allowing more drainback will produce lower output pressure when the level of the pan is low.
If we can assume that we are only pumping oil (a non-compressible liquid) and that a fixed volume of oil is pumped per rotation, assuring us that the same volume of oil is pumped through the engine no matter what the oil level is, then we must attribute a change in the oil pressure reading to lower oil viscosity.
What can lower oil viscosity?
Compositional breakdown occurs as the oil is contaminated with moisture and hydrocarbons from blow-by, and as the oil polymers themselves break down from heat and shear.
The same amount of blow-by per firing cycle on a hot motor enters the crankcase regardless of the oil level, and the motor relies on being run hot in order to evaporate the moisture (but not the hydrocarbon contamination) from the crankcase. The oil is thus broken down and contaminated by time spent normally driving, and the effect of this contamination is worse when the engine spends a greater proportion of its total operating time warming up from cold. When the oil level is low, a smaller volume of oil must bear the same amount of contamination and thermal/shear stress that causes its breakdown.
It is expected then that a low level of used oil will produce lower oil pressure than a low level of new oil. It is also expected that the effect on oil pressure of being low a certain amount of oil will be more pronounced on engines with a smaller crankcase capacity. But it is also expected then that on any engine, being low on oil will not produce an effect on oil pressure if the oil in the crankcase is new (not contaminated or broken down in any way).
Oil viscosity is reduced when its temperature is raised. Circulated oil accumulates heat as it is sprayed on the cylinder walls, but radiates it through the oil cooler and oil pan, until an overall thermal equilibrium is reached. It is important to note that if a particular volume of oil was constantly recirculated, it would reach equilibrium at the temperature of the engine, while if it was seldom recirculated, it would reach equilibrium at the external temperature of the oil pan. Thus, the temperature of the oil has a high negative correlation with the time that the oil spends sitting in the oil pan.
The average time that a just-circulated volume of oil spends in the oil pan is determined by the amount of other oil that is in the pan below it. The other oil is cooler than the oil that has just circulated, so the just-circulated oil radiates its heat through the other oil and through the pan while it waits to be recirculated.
So, it appears safe to state that a low oil level is a causal factor in the increased temperature of the oil, and thus lower overall viscosity and lower oil pressure. This effect, unlike the effect of contamination and breakdown, should be present even if the crankcase is filled with new oil. Again, it should be predictable that the effect will be more pronounced in a smaller crankcase for a given volume of missing oil. But, while it is a causal factor, how much of a factor is it really? After all, the cold pressure and warm pressure in a full crankcase hardly varies enough to register on the needle. It seems like the temperature variance just doesn’t cause that big of a hit on the oil pressure.
Revisiting the air trapped in the oil, we would expect the same proportion of air trapped in the oil if the oil level is low, because the same surface area where air could potentially escape exists no matter what the oil level is. (Of course, if the oil level is far too high, a disproportionate amount of air will be trapped, but that is due to the crankshaft frothing the oil and not due to normal oil circulation.)
So if an engine is filled with a too-low level of new oil, we should expect a lower than normal oil pressure reading at all times due to:
- Small leaks in the oil pickup that would be masked by a higher oil level
- An amount of drainback relative to how worn the oil pump is
We should expect a lower than normal oil pressure reading at operating temperature — but possibly a vanishingly small difference — due to:
- Temperature modification of the oil viscosity
We should expect a lower than normal oil pressure reading after a fixed number of miles of usual driving due to:
- Accelerated contamination of the total volume of oil (occurring at a rate proportional to the percentage of time that the engine spends running below operating temperature)
- Accelerated rate of polymer chain breakdown from shear and thermal stress. Thermal stress may not be a major issue. Since a low oil level causes an increased equilibrium oil temperature when the engine is running at operating temperature, we can also predict that the thermal breakdown of the total volume of oil when running at operating temperature will occur at a faster rate when the oil level is low. But as reasoned above, the equilibrium temperature probably will not rise enough to significantly affect the oil viscosity, so the effects of any increased thermal stress on the long-term viscosity of the oil may be negligible.