Prius rate of oil burn depending on dipstick level

My guess is the hydro forces on the piston rings from below are greater when the oil level is higher... Unless you have an oil leak and then it would be an oil leak up higher in the crankcase.

And I bet some people will say to check your PCV valve, but I think you'd have other problems with how well your engine works if that $8 valve wasn't working right, but people are obsessed with changing that valve because they're cheap and 1/2 century ago was a common source of trouble because engines ran way dirtier back then, but not anymore.

 

Probably not a linear level to volume correspondence. Measuring level rather than volume. If the oil pan is wider at the lower level more volume gets burned per mm.

 

Hi Joao, setting aside variables such as driving patterns and weather conditions for the time being, there are three main factors which affect the rate of motor oil level change after an oil change: volatility, viscosity, and dilution.

This is a typical effect of the change in oil volatility after an oil change. Volatility of motor oil is typically measured using the Noack test.
From Wikipedia:

Note the first part of the last sentence. Motor oil is almost always a mixture of different oil molecules, having their own different volatilities. The portion of the mix with higher volatility tends to evaporate sooner, leaving a higher portion of lower volatility oil in the mix, which evaporates more slowly.

Due to this effect (excluding other effects for the moment), the oil level tends to decrease most rapidly right after an oil change, and decrease less and less rapidly as the oil evaporates.

Part of this burn rate decrease can be explained by decreased volatility alone, but as mentioned in the Wikipedia article, the evaporation tends to lead to an increase in oil viscosity. Also, gradual chemical degradation of the oil and the viscosity modifier additives tend to slowly increase the oil viscosity. Generally, higher oil viscosity tends to decrease the burn rate, but how much of a role that plays depends very much on the design and condition of the engine.


Especially during cold starts and in cold or humid weather, water and fuel vapor condense inside the engine and become mixed and dissolved in the motor oil. This oil dilution with water and fuel increases the volume of the oil, counteracting the lowering of the oil level. The amount of dilution depends strongly upon driving patterns and weather conditions. in extreme cases, such as very short trips and many cold starts, it can even cause the oil level to rise.

Normally, when the oil gets hot, the water and fuel start to evaporate back out of the oil, so that the dilution rarely gets above only a few percent. For a certain driving pattern and conditions, it typically eventually reaches and oscillates around a certain equilibrium point. So the question is how long after an oil change does the dilution continues to build up and affect the oil level. But under normal conditions, the effect would probably not be very large.

 

Hi RJ, Good point. It most certainly is not a perfectly linear correspondence, judging from the shape of the oil pan.

I have been tracking the motor oil level in my Gen 2 Prius for quite a while, measuring on the dipstick before and after refilling with a measured volume of oil to various levels between the dipstick marks.

When I calculate the volume difference based on a linear extrapolation of 1.5 liters to 43 mm between the dipstick marks, and compare that with the measured volume of the oil actually refilled, the error is usually less than around 5% of the total capacity. That is also including the volume measurement error using a plastic measuring beaker.

So, from my personal experience, I would think that the lack of precision of measuring with the dipstick is not bad enough to completely account for the change in the rate of oil level that Joao observed. But still, it's good to keep in mind that measuring with the dipstick is not very precise.

 

I was thinking similar, only thing: I think the bottom of oil pan rounds inwards, gets narrower, towards the bottom? Still, it could be something similar; I think the oil consumption would tend to be the same all along. Or even worse as it gets low.

 

Why are you only filling it up to 75% of the dipstick? Of course its going to consume more oil THE OIL LEVEL IS LOW TO BEGIN WITH.

Its a tiny motor it needs every drop of oil. In fact your better off filling it up slightly over the full line.

Move up to 40 weight oil it will help the oil loss.