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Your Engine's Most Important System
The most important system to the "health" of your engine is the lubrication system. Regular oil changes (every 3000 to 7000 miles, or every 6 months if it's a vehicle that's not driven often) with the proper type of oil for your car will assure you get the maximum possible mileage and performance from your engine. Low mileage "granny cars" that only make short trips build up acids in the crankcase from condensed blowby gasses. That's why you should change oil every 6 months even if you haven't driven a lot of miles since the last change.
where to go for an oil change
You can change your oil yourself or take it to a "quick lube" place, but you'll really get the most benefit from letting your regular mechanic change your oil. Why?
Well, first because he has a lift and all the proper tools to do the job right. Do you really want to crawl on the ground to save 5 or 10 bucks? Second, there are a lot of things a trained mechanic will notice that you might miss (if for no other reason than that you don't have the car up in the air so you can walk around under it and look around). He'll also grease the front suspension: an item often ignored by do-it-yourself folks.
Quick Lube Services
So what about those 10 minute folks? Well, they do have a multipoint inspection, but who's doing it? Their "techs" are paid only slightly above minimum wage and they work in a hot pit all day. These are not the best mechanics in town. Also, often they will use cheap filters, and sometimes the oil they use is not exactly the right viscosity: substituting 10w-30 oil for 5w-30 oil, etc.
HOW OFTEN SHOULD YOU CHANGE YOUR OIL?
The answer is not as simple as it used to be! Many oil manufacturers (mostly the synthetic guys) are touting obscenely high oil change intervals. Many vehicle manufacturers are recommending 5000 or even 7000 mile oil change intervals. Of course the old "pat answer" used to be every 3000 miles. If you do a lot of highway driving that would have you changing oil every time you turned around!
A few basics:
why oil needs to be changed
Oil should be changed for several reasons: first, oil will break down chemically over time and lose its lubrication properties. Multiviscosity oils can lose their ability to flow more freely (than single viscosity oils) in cold weather. All oils (single and multi viscosity) can "thin out" with time and lessen their ability to lubricate. Heat accelerates these changes, therefore oil would last longer in a moderate climate than in a hot one.
The reason synthetic oils last longer is they have a "more pure and stable" molecular structure that is more resistive to breakdown than traditionally refined oils.
As an oil breaks down it combines with combustion chamber residue from blowby gasses to form sludge. High detergent oils dissolve this sludge and hold it in suspension. In addition, as an engine wears small pieces of metal, rubber, and plastic end up in the bottom of the oil pan. Believe it or not, the "granny car" which is "short tripped" will have a lot more acids (condensed from blowby gasses) mixed with the oil than a car which is driven every day! I rebuilt one engine with a thrown rod: (old lady, once a week to the store 3 miles away) The rod bearing had been eaten away by acid deposits!!! Ony 40,000 miles on that motor! Thus on low mileage cars I recommend changing the oil every 6 months, no matter how many miles on them!
So anyway, the oil should be changed to get rid of all the "crap" that gets dissolved in it, even if the oil itself is still chemically OK.
SO HOW OFTEN SHOULD YOU CHANGE YOUR OIL?
I don't go by either miles or time on my oil in my cars! I go by "feel" and "appearance"! When I check my oil (you should do this regularly: how regularly depends on how much oil your car uses: You'll "get a feel" for its "oil habit!") I wipe the dipstick off with my fingers. I then rub my fingers together and "feel" the oil's viscosity. Good oil feels slick: you can't feel the ridges in your fingerprint. Really bad oil feels almost like water: your fingerprint ridges are really obvious. "Feel" the oil right after you change it to get an idea how good oil feels. Then when you check it, if it feels thinner than that, change it!
Step 2: reinsert the dipstick, pull it out to check the level. You should be able to read the markings on the dipstick through the oil. The oil should be a light caramel color: about like pancake syrup at the very darkest. If it is darker than that, it's time for a change.
Low Mileage Vehicles
If you don't put a lot of miles on your car you need to go by time rather than miles for oil changes. Blowby gasses in the crankcase condense to form all kinds of nasty acids that can literally eat up your engine. On a car that is driven every day these condensates get "boiled off" by engine heat, but on a car that only takes short infrequent "trips to the store" these condensates can build up in the engine. So if you have a "granny car" that only drives a few hundred miles per year you should change the oil every 6 months to a year regardless of mileage.
Can you change your oil too often?
OK, let's say you're a fanatic with a lot of money and want to change your oil every 3000 miles whether you need it or not: is there any reason not to do this?
Yes there is! Each time you change the oil the oil pump "loses its prime" and the new oil filter is empty. Even if you pour some oil in the new filter you'll leave an air bubble in there. When you start the engine, it will have to run for a short time without oil pressure: that's an automotive heart attack! (see my article on your oil pressure light or gage) Before the oil can reach the bearings, the air must be forced out through the oil passages, and air doesn't lubricate motors very well!
This is really not a huge big deal: the film on the bearings works for a surprising length of time even with low oil pressure, but why do it any more than you have to?
A WARNING: IF YOUR CAR IS UNDER A WARRANTY!!!!!!!!
If your car is under warranty you may void this warranty if you exceed the oil change intervals recommended by the manufacturer!!!!!!!! Keep records and reciepts for oil changes if you are under a warranty!
Related topic: What to do when your oil pressure light comes on