This is probably one of the biggest scams of the oil industry. Some how they have convinced the public that they need to change their oil every 3000 miles, regardless of whether it is synthetic or not and regardless of the filter you use (and how often you change it). Granted, this may have been the case 50 years ago, but we have changed our opinions about other things from back then so why are we still hung up about this? The problem is, that unless you know the facts, you tend to let an ďexpertĒ tell you what to do. In this case, the experts are usually quick lube places or your dealer, who despite their profession and knowledge donít want to tell you the truth. They want us to think that we need a change every 3000 miles so they can sell us more oil and services. Can you imagine what it would do to the oil industry if all of a sudden only half the oil used in cars was needed? Iím not saying that I am an expert on oil. I am saying from the information that I have gathered over the years and from my personal experience running synthetic oils and filters in all my cars (performance cars and not), I believe that we can at least double our interval by using a synthetic oil and a synthetic filter. On top of saving time you also get superior performance and protection with a synthetic.
It is important to note that there are a lot of reasons why we can extend the interval, and not all of them have to do with the oil. The oil filter plays a huge roll and is a complicated beast. There are many different kinds and it is critical to pick the right one. The filter is what keeps your engine from wearing out. It filters out particles that get into the oil from the engine and other places. There are many important parameters for oil filters but two big ones are: the amount of flow they allow and the smallest particle size that can be filtered out. Unfortunately, there is a compromise between the two. You canít filter out everything and have the oil flow through the filter quickly. In situations where you want your car to last forever, you should buy the filter that can clear out the smallest particle but be aware that you are giving up performance. In applications where performance is more important, you will want more flow and are willing to give up some engine life for it. In the end though, the synthetic oil filters are superior regardless of which parameter is more important to your driving style and goal.
In an ideal world synthetic oil never really breaks down like regular oil. The reason we change it is because the filter can only do so much, and other things get into the oil which canít be filtered out by the filter. The filter has a set life, and always will. It is essentially a sponge that sucks in all the particles. After a while, there is no more room for the filter to suck in new particles and it gets saturated. Thatís why it is critical to change the filter often for maximum protection.
In cases where particular types of synthetic oils are used, changing the oil filter every 5k miles can allow you oil change intervals of up to 20k miles. Many new luxury cars are starting to adapt, along with their dealers, to this capability. Mercedes and BMW have indicators that tell you when to change the oil, and in many cases that can be around 10k miles. The Corvette has a system that looks at your driving style and environment and then tells you when to change. In the case of the Vette, many owners go at least 7k miles per oil change. Of course, regardless of what your ownerís manual says, the oil change shop will always put the sticker in your car to return in 3k miles.
It is important to note however, that driving conditions play a big role in this. If you constantly drive in extreme weather and only run the car for short trips, you will need to change more frequently. It is very important for the oil and other fluids to come to the right temperature. For those of us in New England, the winter can be particularly bad. Since it is very cold we tend to take many more short trips than in the summer. This does not allow the oil to come to temperature and burn off a lot of contaminants. This is often seen in owners manuals. For example, the Honda Accord 2002 manual states oil change intervals of 7,500 miles on regular oil (not bad) but 3,000 miles if you are in extreme climates and drive the car for under 8 miles per trip.
In general, I have found that you can go at least 6k miles with synthetic without any problem and on the same filter. This is what I do in my cars that do not have change indicators. Of course this turns out to be the break even point in terms of money over regular oil because synthetic is about twice as expensive. If you change your filter frequently, you can greatly extend this and now itís worth it.
Of course many of us use synthetic for the performance advantages regardless of the change interval. But itís nice for once that something you do for performance (and performance here means many things like increased fuel efficiency, better engine protection, lower engine temps, more horse power etc.) can also end up costing you less, saving you time, and protect your engine better.
There are some great resources for further information on this topic. One is The Motor Oil Bible by Michael Kaufman which can be downloaded from the web. He goes through all the details of synthetic oil and extended oil change intervals and has a great comparison chart of all the synthetics with their strengths and weaknesses. In addition, although often spun to their advantage, are tests performed by the oil manufacturers on their websites (www.mobile1.com, www.redlineoil.com). Another good website is www.bestsynthetic.com. This site does promote a particular oil, but at least goes through the important parameters that oils should meet and has discussions on the benefits of synthetics.
Of course this is all my opinion. But look at it this way, if a Vette that is driven hard can have a change interval of 7k miles, then other cars driven normally can do at least that. Check out some of the references listed above and you will see that this "scam" is being uncovered more and more every day.