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!