What is the importance
Not only does engine oil lubricate, but it also somewhat helps cools an engine. but that heat breaks down the oil over time, reducing it’s capacity to absorb heat, making it more viscous, and ultimately volatile. Oil that cannot absorb more heat starts smoking. particles from normal engine wear end up in oil as well, ideally trapped in the filter. this somewhat reduces oil flow. some older engines will have a tendency to leak or burn oil, reducing the amount in the engine. (this is considered relatively normal in older engines).
changing the oil makes sure the proper amount is in the engine, that it’s at full lubricating and heat absorption abilities, and that it’s clean. in terms of basic level maintenance, the worst thing to a car is not having its oil changed at proper intervals. Not changing the oil one of the few things a car owner can do that will lead to catastrophic engine failure with any amount of certainty.
Other answers have more than capably covered how important it is to change the oil so I will mention a few other items to do with the subject.
One is the formation of a substance called sludge in the engine. Before detergent oils were introduced this was a huge problem and will ruin an engine. Articles used to be written about the problem back in the 50’s. There are many reasons for the formation of sludge but timely oil changes keep it at bay. Oil sludge
The use of synthetic oils such as Mobil 1 and Amsoil can extend the life of your engine as well as increase power and fuel economy. Synthetic oil
The most important thing engine oil does is to prevent most (though not all) metal-to-metal contact inside your engine. The pump forces oil at high pressure into all the main bearings – crankshaft-to-block, rod-to-crankshaft, camshaft-to-block, etc – to make the parts ‘float’, similar to the puck in air hockey. The oil’s viscosity plays a huge rule in this department, controlling how readily it will flow into these ultra-tight clearances as well as how well it resists getting squished and squeezed out.
Over time, the heat and pressure of simply doing its job causes oil to eventually break down, altering its viscosity and causing it to be less effective at its job.
One of the places oil can’t prevent metal-to-metal contact are the piston rings. The combustion cylinders are a literal hell for engine components, the extreme heat causing the greatest heat expansion inside an engine and combustion itself can force its way past the piston rings to blow burned hydrocarbons directly into the engine oil. The piston rings grinding against the cylinder walls and the brief metal-to-metal contact when you first start the engine produce microscopic fragments of metal.
Engine oil filters come in various grades, both in design and materials used, but they all work to remove all these impurities from your engine oil. There’s a great deal of variegation between different types of engine oils, the various synthetic additives they include, and countless other factors (such as city driving vs highway driving) which can influence the performance and longevity of engine oil. But, no matter what, the oil filter will eventually look like this:
By this point, the oil pump can’t draw enough oil through the filter to keep all the journals lubricated. In addition to creating a lot of metal-to-metal contact and causing serious damage to your engine, the extra heat from the friction is burning the oil from the inside-out. The more damaged the oil becomes, the less it can do to protect your engine.
Changing the oil regularly (which includes replacing the filter) ensures that the engine has an adequate supply of oil (topping off regularly helps), that the oil itself is up to the job it’s intended to do (right viscosity, little breakdown), and that it’s free of carbons, metal, and other junk which inevitably wind up in your oil. A good quality oil filter can go a long way to prolong how well the oil can do its job but, inevitably, neither the oil nor the filter will be able to perform adequately.
There are two methods one can use to confirm what shape your engine oil is in.
First is a basic visual inspection – for the most part, if you can see through it it’s perfectly fine to keep using it.
So long as you have the dipstick out, after checking both level and color indicators (the dipstick cools quickly), rub a little between your thumb and index finger. If you can FEEL grit in the oil – no matter what color the oil might be – something’s going on that you need to investigate.
If this shows up some time after an oil change (around the time the oil looks like 3 above), that’s a pretty good indication that you need an engine flush. Even the best-maintained engines in the world will inevitably accumulate gunk in out-of-the-way crevices and an engine flush will do a good job clearing out most of it.
If it shows up very soon after an oil change, either you’re WAY overdue for an engine flush or you’ve got serious mechanical problems liable to leave you stranded alongside the road before long. Check with a competent mechanic ASAP!
According to Valvoline Oil Research: In 5,000 KM of driving, combustion gases that blow pass the piston into the oil lube area – will pass 100 gallons moisture, up to 5 gallons gas, a coffee cup of burnt gas and oil, and a coffee cup amount of acids – into your oil. Did you know that?
Most of that combustion is sucked back up – and re-circulated for re-burning. But not all!
Good oil will hold a cup or so of sludge. As the oil goes past the 2,500 KM mark, city driven oil starts losing its ability to flow as it should. And, its getting dirty. Its also starting to resist being whipped up by the crankshaft. It has burnt gas and oil flakes in it – the darker color, and its starting to turn acidic – which attacks your motor rubbing surfaces, but also helps thicken the oil – as well as weaken its lube strength. Becoming less fluid, near 8000 KM, it begins to takes a bit more fuel to burn to overcome its oil pump pushing action, and its crankshaft flogging. Its got not much good stuff going for it by then.
We are not considering oil leaks, oil burning, valve tapping, friction growth, and power decline as a factor city driving negatives in all this – but those conditions appear more so in city driven vehicles, I believe. City driven vehicles make me and others in the additive business a lot of income. And mechanics too!
Anyway… What is a city driving anyhow, as regards to oil change intervals?
Are you a city driver?
City drivers – are those classified that do not drive one hour continuously, at 60+ MPH, daily, 5 days a week. If that is not close to your driving style you do well changing you oil by 10,000 KM. 5000 – 6500 may be better.
Those running the highways can go 20,000 KM – on oil changes, and may run up 20,000 in a few months. Their oil will be cleaner at 20,000 KM – as less blowby occurs, and the emission system is working at peak efficiency, constantly doing highway miles. Oil contamination is low.
So, with this info, when will you change your oil? And why?