You may wonder why oil doesn't seem to last as long as other fluids in your car. You may also have wondered how there could possibly be so many different kinds of oils. After all, whether you come in 500 miles before the oil change is due or 3000 miles after it's due, the car seems to drive the same. Well your wait has come to an end.
Oil is made up of a foundational compound (called the “base oil”) and additives (called “additives”—I know, I know, don't quit my day job). What additives are included in the oil depend on what vehicles the oil is designed for. A large portion of research and development has been devoted to fuel efficiency and decreased emissions.
It wasn't uncommon for passenger cars in the 70s and 80s to call for a 10w-40 oil. Now many passenger cars recommend using a 0w-20 oil. You don't have to understand much about weight ratings and viscosity to know that's a lot less thick. One reason we use thinner oils is that less viscosity CAN translate to decreased resistance to moving parts and in turn increased efficiency.
I'm not looking to dive deep into each of the following reasons motor oil degrades but I hope that by sharing you'll be able to research more if you're interested: oxidation, high heat, moisture, viscosity loss, fuel dilution, and additive loss to name a few. Essentially, a whole lot of factors affect oil wear. Most people don't care enough to know the exact moment their oil is going bad, so manufacturers issue a service interval that is good for most people.
This brings up the point that no universal standard can account for all unique driving styles. Habits that cause oil to degrade quickly include, but are not limited to, taking short trips in the vehicle frequently, accelerating hard while vehicle is warming up, and towing or requiring full power of the vehicle often.
Regarding oil's short lifespan, no other fluid in your car interacts with other fluids (and under such harsh conditions too). Usually the only completely sealed fluid is the brake fluid. The most common interaction is oil and gasoline. Oil is burned in trace amounts in the combustion chamber and what isn't burned returns very hot to other parts of the motor. Oil travels through metal-on-metal areas that produce shear forces that break down the oil and thus reduce its ability to lubricate, cool, and protect. Finally, oil is exposed to more air and moisture than any other fluid which can cause oxidation and dilution of the oil.
Different manufacturers recommend different specifications for oil and every aftermarket oil manufacturer wants a piece of the pie. Marketing is also to blame for oil quality confusion. Just remember, changing your oil is probably more important than agonizing over which oil to use. More and more research is being worked through to find the best additives but you can usually trust your local mechanic to pick a good oil for your vehicle.
Source & Influence: Amsoil Mailer, Jan 2020, Amsoil Inc., All Rights Reserved