Optimum oil level=less resistance
 

I never thought about this until I tried to explain the results I was coming up with. I have a 01 Corolla which as most know tend to burn a little oil with higher mileage. I noticed that topping the oil off at fuel stops on highway driving trips cause a noticeable drop in MPG.

The oil never gets low enough for the check oil light to come on, but the level on the dipstick is about half way down. I fill to the full mark. My question is the minimum oil does the engine actually needs to achieve full lubrication. Read that question again before you reply with a biased opinion and no factual information. THIS IS A TECH THREAD NOT UNSUPPORTED OPINIONS.

As far as lubrication goes the minimum level is a matter of whether the oil sump sucks in any air while the engine is operating. Other things need to be considered though. Oil also acts like a coolant to some degree. Less of it means it will get hotter because it will spend less time sitting in the pan cooling off. The hotter it gets the sooner its lubrication properties will break down.

From the factory, the sump and oil level are designed to supply uninterrupted oil flow through the engine for extended periods at wide-open-throttle up to red-line or limiter imposed rev limit of the engine.
I have no idea what temperature range this is good down to.

I don't think anyone can give you an exact answer for two reasons.

First "to achieve full lubrication" - but under what conditions?
hypermiling? dirt-track racing?

Second, sump efficiency differs with design, and drain-back characteristics can be affected by many variables such as sludge build-up, core-shift in block or head, or the alignment of holes in the head gasket. Bearing leakage changes as clearances change, and that will obviously affect oiling requirements, pressure, and drain-back.

This is somewhat unrelated, but I know of a few drag racers who have got away with making WOT passes while 1.5 quarts low (3-quarts remaining) without experiencing pressure loss or problem. From this they saw a performance increase of between 10 and 15 HP based on their trap speeds at the end. I do know that they were using standard un-baffled pans, but one of them used a windage screen.

I do know that oil volume requirement is at least in part a function of engine speed, and thus low RPM cruising (hypermiling) would have less demand than what the system is designed to cover.

All true points.



As far as the original question goes, the safe operating range of oil level is anywhere between the two marks on the dipstick. You do not need to fill it to the full mark every time. People do this because some cars, like yours, use oil and it allows them a certain amount of time between checks.

The owners manual will tell you how much oil it takes to get from the low mark to full mark, it is typically 1qt but can vary from manufacturer to manufacturer.

You can also run below the low mark, but it is not recommended. If you continually drive your car at low RPMs, I would say that you can get away with less oil, because it will be pumped up to the cylinder head at a lower rate, so drainback time to the sump is not an issue.

However on some engines, drainback can be an issue during prolonged high RPM usage. The sump can essentially empty itself into the heads faster than gravity can drain the oil back down.

Identical post times with near identical answers :thumbup:

If I could have any oil system I wanted RIGHT NOW, it would be a dry sump. No oil to splash around and no worry about needing baffles.

Does the oil route after the sump affect things ? As I understand it oil passes through the filter after the sump and this acts as a kind of small 'reservoir' for short dry periods (non return type). Obviously anything more than a couple of seconds of sucking air is pretty bad.

Also why don't road engines have dry sump systems ? Seems like less fuss.

Shorter lifespan
 

Driving with less oil in the engine wears out the oil quicker, so this willf orce you in to change oil more frequent.

If we assume, that your car takes 5 liters of engine oil, and theres a 1 liter difference of the full mark and min. mark on the dipstick, running all the time whit the oil at the min level, will reduce your oils life time with at least 20 percent maybee more.

The reason beeing, that the oil pump will circulate the same amount of oil in the engine when you drive a given distance, but now you only have 4 liters so the oil will circulates 20% more, and thus wear out much earlier.

I belive it a bad idear. Just dont over fill, but up to the max mark.

While what Jyden said is true about the oil being used more, I don't think it will be a problem. I've routinely run 10,000 miles on conventional oil in some of my cars. I do an oil analysis to make sure all is well and while this is stretching the life of the oil, it is usually fine. If you run synthetic, you have even more life in the oil.

Jyden: you have no, and cannot obtain, any proof of that.

In fact I have read from the manufacturer that they do NOT recommend always topping off to the full line. Their position is to NOT add until the level drops down to the ADD line.

Now, one possible reason for rock's observation is that when at the full line his engine has what is called windage, which is caused by the crankshaft physically dipping into the oil and whipping it around. Windage is bad not only because it takes horsepower; it whips the oil into an aeriated froth which then does not do as good a job at providing a lubricating film.

Go ahead and run it down to the add line. Or even start things off at the add line. It is possible to successfully run engines below the add line too as the drag racers have found out but for peace of mind I wouldn't go there.