You've heard the saying, your mileage may vary? The same applies to oil and filter changes, too. The intervals (time and mileage accrued) are general recommendations that may not apply in your specific circumstances.
Variables that affect change intervals include: How old the car is; how often the car is driven; how the car is driven.
Let's look at each.
* How old is your car?
Older cars usually need more frequent oil/filter changes. In part, because their design requires it - but also because of their age. Older/high-mileage car engines are more likely to have varnish and sludge build-up than newer/lower mileage engines. More frequent oil changes will help clear out the gunk - and help keep the engine clean. If your car is really old - and has a carburetor - you may need to abide by the old three months/3,000 mile standard.
Carburetors tend to weep raw gas (which is a solvent) into the engine, which dilutes the oil. Carbureted engines also tend to not have ideal air-fuel mixtures; often, they run a little rich or a little lean. A little rich (more gas than ideal) and carbon and such tends to accrue, which contaminates the oil. A little lean and the engine may run hot, which makes life harder on the oil (and additive package) which shortens the oil's useful service.
The advent of computer controlled engine management and fuel injection ameliorated both these issues - and that's one of the big reasons why advertised oil/filter change intervals have lengthened over the past couple of decades.
* How often is the car driven?
It's kind of counterintuitive, but - generally - the less you use your car, the more often you probably ought to change the oil (and filter). There are two reasons why. First, a car that sits a lot is more vulnerable to the accumulation (inside the engine) of moisture resulting from condensation. Oil and water don't mix well, as you know - and water contamination can lead to rust inside the engine, which is something you don't want to happen. Running the engine heats it up and burns off moisture but if the engine is run only briefly and occasionally, it might not get hot enough to burn off the moisture and as it cools down again, more moisture accumulates. You may see where this is headed.
Older cars (especially those made before the adoption of fuel injection and computer engine controls, so circa mid-1980s and prior) are especially vulnerable and should have their oil/filter changed at least as often as the original factory recommendation and probably more often, if they're left to sit for weeks at a time (by now, most of these '80s-era and older cars are recreational/hobby cars and not used regularly almost by default).
There are specially formulated oils (Amsoil, which I personally use and highly recommend see here for more) designed specifically for older car engines, both their type (the oil contains the special anti-friction additives needed by these older engines) as well as the occasional use they tend to see.
* How is the car driven?
A race car's engine gets maybe a few hundred miles of life out of fresh oil (and often, the engine itself only lasts a few hundred miles - or less - before it's torn down and rebuilt). Hard use = shorter life. But hard use - in a street car - can mean things you might not expect.
It can mean stop-and-go driving, periods of prolonged idling... the kind of driving most of us who commute do pretty much every day. It doesn't feel like hard use, but it is - and (check your vehicle's owner's manual) may mean shorter-than-advertised change intervals.
The tricky thing is that the manufacturers like to tout the longer/best-case intervals as a selling point. It's not unlike the gas mileage figures they tout - which are also best-case figures.
The fact is that your mileage may vary - in both cases.
Incidentals:
* Many late-model cars have oil/filter change reminders and/or oil-life monitors. Some are "smart" ... and some not-so-much.
It's important to know which your car has before you rely on it as the basis for deciding when to change the oil. The smart systems have the ability to sample the oil and monitor its condition. It's kind of like having your own lab analysis to go. These systems will let you know when it's time to change the oil - whether sooner or later - depending on the oil's actual condition, based on your actual driving habits.
The dumb systems are simply mileage meters that trigger a "change oil" light after say 5,000 miles of driving (or whatever it was set for). In fact, the oil may have been tired at 3,000 miles - or good to go for another 5,000.
So, how to know which system you have? The information should be in your owner's manual. If not, ask at the dealer - but ask them to show you something in writing, from the manufacturer of your vehicle. Don't decide based solely on what the service advisor tells you. He may be a salesman who works on commission.
* Using the "right" oil (and filter).
New/recent model cars can be very finicky about the type of oil they need - and the car manufacturers can be very finicky about warranty coverage if you use something other than the specified weight (e.g., 5W30) or oil that doesn't meet the minimum SAE requirements for the vehicle (see here for more info).
In older cars, you could usually safely use a heavier weight (such as 20W50) in lieu of (or mixed with) a lighter weight (such as 10W40) without worrying about potential mechanical or warranty issues. But they didn't have variable cam/valve timing systems (which use engine oil, typically) or have the extremely tight internal tolerances modern car engines typically have. In a pinch, it's probably ok to top off with say a quart of 5W30 if that's all that's available. But if your engine wants say 0W30 oil, that's what you should use. If you're forced to use something else, it's smart policy to change the oil as soon as possible - and refill it with the manufacturer-specified weight.
* Keep records.
If you do your own oil and filter changes (or even if you have them done by an independent repair shop) be sure to keep receipts and records that state when the work was done (date and mileage) as well as the type/weight of oil and the type/brand of filter used. Be sure this is clearly spelled out on your receipt - not just "oil change" or "five quarts" and "filter."
If you ever have to make a warranty claim for something that's oil-related, this paper trail may be the difference between the manufacturer honoring the warranty... or not.
The manufacturer isn't being shady. It's reasonable for them to deny a claim if the vehicle owner filled the engine with oil that didn't meet their specifications or used an el-cheapo oil filter that failed and then so did the engine. Keeping records and receipts keeps everyone honest. You can prove you maintained the car properly - and they can't shirk paying for a problem caused by poor design or shoddy assembly by blaming it on your not maintaining the car properly.
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