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What Every Motorist Should Know About Oil Changes

Copyright AA1Car.com

How often should you change oil? On most late model vehicles, the factory recommended service interval for changing your oil and oil filter is typically once a year or every 5,000 to 7,500 miles (which ever comes first). Some vehicle manufacturers even recommend 10,000 mile oil change intervals. Others have no recommended service interval but instead rely on an oil service reminder light to let you know when an oil change is due. Some oil reminder lights may not come on for as many as 10,000 to 12,000 miles or more under ideal driving conditions (Note: such extended oil intervals are usually based on using a synthetic motor oil, not conventional motor oil).

For passenger car and light truck diesel engines and turbocharged gasoline engines, the traditional recommended service interval is typically every 3,000 miles or six months (which ever comes first). But this recommendation has changed on most late model cars and trucks.

How Long Should You Wait To Change Oil?

If you read the fine print in your owners manual scheduled maintenance section, you will often discover that many extended oil change intervals (those beyond 5,000 miles) are for vehicles that are driven under "ideal" operating conditions. What most of us think of as "normal" driving is actually "severe service" driving as far as the oil is concerned.


Severe service driving includes:

Frequent short trips (less than 10 miles, especially during cold weather)

Stop-and-go city traffic driving

Driving in dusty conditions (gravel roads, etc.)

Driving at sustained highway speeds during hot weather.

For severe service driving (which is what most of us do), the most common recommendation is to change the oil every 3,000 miles or six months (which ever comes first). Such frequent oil changes might not be necessary in late model low mileage engines, but it is important for older, high mileage engines (those with over 75,000 to 100,000 miles).

For maximum protection, many people change their oil every 3,000 miles or three to six months regardless of what kind of driving they do or the mileage on their engine. Some would argue this is excessive maintenance and wastes oil. Changing the oil every 3,000 miles may NOT be absolutely necessary on a new low mileage vehicle, especially if the vehicle is driven more than 10 miles one-way daily or is used primarily for highway driving or long distance commuting. Changing the oil after only 3,000 miles should also not be necessary if you are using a high quality SYNTHETIC motor oil (which provides much longer life than conventional motor oil). Even so, regular oil changes cost a lot less than replacing an engine!

Newer engines can usually go 5,000 to 7,500 miles between oil changes with no harm. But in my opinion, pushing the oil change interval beyond 7,500 miles with a conventional non-synthetic motor oil is asking for trouble, especially if an engine has more than 75,000 miles on it, or it operated under "severe" conditions as described above. Several vehicle manufacturers (Chrysler and Toyota) who have recommended longer service intervals have run into problems with sludge forming inside some engines, causing expensive damage.

On my own cars, I change the oil every 3,000 miles if most of my driving has been short trips around town. I will wait to change oil until 5,000 miles if most of my driving has been on the highway, and ONLY on my cars that have less than 60,000 miles on the odometer (I have several cars). Following this oil change schedule, I have NEVER had any major engine problems or engine failures in over 30 years of driving! I change my oil myself and always buy a quality name brand oil and filter.

As an engine accumulates miles, blowby increases. Combustion gases leak past the rings (blowby). This allows more unburned fuel and moisture to enter the crankcase. Fuel dilutes the oil, and moisture promotes the formation of acids and sludge. So if the oil is not changed often enough, you can end up with accelerated wear, oil sludging and possibly a premature engine failure!

motor oil, oil filter
Engine lubrication system.


General Motors and a number of European vehicles have done away with recommended oil change intervals altogether and now use an "oil reminder" light to signal the driver when an oil change is needed. On some of these (Volkswagen, Audi, Mercedes Benz, BMW and Volvo), a scan tool or a special service tool is required to reset the oil service reminder light. On others, there is a manual procedure for resetting the oil reminder light.

DO NOT IGNORE THE LIGHT! When the light is on, it is time to change the oil.

The oil reminder systems estimate oil life based on engine running time, miles driven, ambient temperature, coolant temperature and other operating conditions. On some of these vehicles, the light may not come on until 10,000 miles or higher! But keep in mind that most of these engines are factory-filled with high quality synthetic oil, so be sure to replace same with same when the oil on these engines is changed if you are going to rely on the oil reminder light alone rather than a mileage/time interval.


One way to know for sure when the oil needs to be changed is to have the oil analyzed at 3,000, 5,000, 7,500 or 10,000 miles. Many fleets do this to optimize their oil change intervals. A small sample of oil (3 to 4 oz.) is taken from the engine, sealed in a bottle and mailed to a laboratory for analysis. The lab runs a series of tests to determine the properties of the oil and the contaminants that are in the oil. A report is then mailed back with a recommendation as to whether or not the oil is still good.

Having your oil analyzed is not something you need to do all the time. Checking it once or twice should help your determine the optimum oil change interval for the type of driving you do. The cost for oil analysis is usually less than $25, and there are a number of companies who offer such services:

Blackstone Laboratories

Oil Analyzers Inc.

Insight Services

sludge in oil pan and oil pump screen
If the oil is not changed as often as needed,
moisture accumulates in the crankcase and causes the oil
to turn to sludge. This can plug up the oil pump inlet,
causing a loss of oil pressure and possible engine damage!


Regardless of what type of oil you use (conventional, synthetic or synthetic blend), all motor oils eventually wear out and have to be changed (actually, it's the additives that wear out more so than the oil). As the miles add up, motor oil loses viscosity and gets dirty. The oil no longer has the same viscosity range it had when it was new, and it contains a lot of gunk (moisture and acids from combustion blowby, soot, dirt and particles of metal from normal wear). You can't really tell much about the condition of the oil by its appearance alone because most oil turns dark brown or black after a few hundred miles of use.

The oil filter will trap most of the solid contaminants, and the Positive Crankcase Ventilation (PCV) system will siphon off most of the moisture and blowby vapors, if the engine gets hot enough and runs long enough to boil the contaminants out of the oil. Even so, after several thousand miles of driving many of the essential additives in the oil that control viscosity, oxidation, wear and corrosion are badly depleted. At this point, the oil begins to break down and provides much less lubrication and protection than when it was new.

If the oil is not changed, the oil may start to gel or form engine-damaging varnish and sludge deposits. Eventually this can cause your engine to fail! Oil sludge can damage engine bearings, piston rings, cylinder walls, valve guides and lifters.

Driving Green: Environmental Considerations of Extending Oil Change Intervals

Extending the interval between oil changes can save money by reducing the quarts of oil that have to buy (and filters), but it also reduces the quarts of waste oil your driving generates over time. Most engines hold four to five quarts of oil, and a few hold six or more quarts. Below is a listing of how much waste oil your engine will generate by changing the oil at various intervals over 100,000 miles of driving.

4 Qt. Crankcase Capacity
Change oil every 3,000 miles = 33 oil changes = 132 quarts (33 gallons) of waste oil generated
Change oil every 5,000 miles = 20 oil changes = 80 quarts (20 gallons) of waste oil generated
Change oil every 7,500 miles = 13 oil changes = 52 quarts (13 gallons) of waste oil generated
Change oil every 10,000 miles = 10 oil changes = 40 quarts (10 gallons) of waste oil generated

5 Qt. Crankcase Capacity
Change oil every 3,000 miles = 33 oil changes = 165 quarts (41 gallons) of waste oil generated
Change oil every 5,000 miles = 20 oil changes = 100 quarts (25 gallons) of waste oil generated
Change oil every 7,500 miles = 13 oil changes = 65 quarts (16 gallons) of waste oil generated
Change oil every 10,000 miles = 10 oil changes = 50 quarts (12.5 gallons) of waste oil generated

6 Qt. Crankcase Capacity
Change oil every 3,000 miles = 33 oil changes = 198 quarts (49.5 gallons) of waste oil generated
Change oil every 5,000 miles = 20 oil changes = 120 quarts (30 gallons) of waste oil generated
Change oil every 7,500 miles = 13 oil changes = 78 quarts (19.5 gallons) of waste oil generated
Change oil every 10,000 miles = 10 oil changes = 60 quarts (15 gallons) of waste oil generated

Waste oil is not necessarily a bad byproduct of driving. If you have your oil changed at a quick lube shop, repair shop or car dealership, the waste oil will be recycled. If you change your own oil, you should take the used oil to an auto parts store or other collection point for recycling.

Never dump used motor oil on the ground or pour it down a storm sewer! The oil will contaminate ground water, and it can kill plants and aquatic life.
If you can't find a place to recycle your used motor oil, pour it through a coffee filter, blend it with heating oil (the more dilute, the better) and burn it in an oil heater.

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