[PA 4WD]

I have engineer friends who are part of engine design teams at a couple different car/truck manufacturers, and I have engineer friends who have worked in auto racing and engine building, who have taken apart more engines than they can count.
They all agree that this oil change issue is one that affects engine longevity. Generally, for those people who don't keep their vehicle longer than 60K or 100K miles, it won't matter to them. But for those people who plan to keep their vehicles beyond the warranty miles, and for those people who buy used vehicles with high miles (60K+), it can matter a lot.
I've seen some of the engines that have been taken apart, and I've seen lots of photos of the metal particals/shavings and the internal engine areas that got close examination. Its very clear, and easily seen, that extended interval oil changes (such as the 5K-10K mile intervals) are damaging to the engine in terms of wear. The amount of wear varies a lot depending on a number of factors including heat, and rpm's, and whether the engine is able to achieve proper operating temperature during driving cycles (short drives from engine on to engine off are often described as extreme duty, and more frequent oil changes are usually recommended).
Those engines that have more frequent oil changes (such as every 3000 miles, or even more often) tend to show much less wear.
But does that "wear" mean that your engine won't run? No, it just affects your engine's efficiency, power, and gas mileage, and eventually (at high miles) will lead to the need for a rebuild because of oil consumption or oil burning, etc.
Without frequent oil changes, quite a few internal engine parts (such as piston rings and valve guides, etc) will wear down more quickly, which results in the compression in the cylinders dropping off sooner, and you'll get more oil blow-by over time (meaning that you're engine will be drinking more oil).
My engineer friends also agree that the early oil changes are some of the most important ones because those are during the "break in" period. They tell me that its called "break in" for a reason, and thats when the most metal particles get in the oil. The break in period apparently varies depending on your particular engine and how it was designed etc. Most mfr's say their break in period is around 1K miles.
Again, the consensus among my engineer friends is that if you are planning to keep your vehicle well beyond the warranty period and into the high mileage numbers (such as over 100K miles), or if you simply want to maintain the maximum possible power and gas mileage for the 60K miles that many people keep their vehicles for, then its a good idea to change the oil and filter at the 500 to 1000 mile point, and then again at the 3000 mile point. How often you change it after that would depend on what type of driving you do (short drives or long drives), how you drive (high revs, or grandma slow, etc), and on what type of oil you use (synthetic will last longer and tolerate higher temperatures than dino oil). Their recommendation for most people is that they stick to a 3000 mile oil change interval, after those initial "break-in" oil changes.
Another few points from my engineer friends, which I would have to agree with:
Many people do not check their oil level, just as they don't check their tire pressures. If you drive 5K or 10K miles between oil changes, its not uncommon to be down a quart or two (or three or four) by that point, especially if you didn't change the oil during or after the important break-in period, and if you do a lot of high speed (high revs) driving. So although the engine oil itself might still be good after X number of miles, there is a question of whether you still have the proper amount of oil present in your engine after so many thousands of miles. Being low by a couple quarts of oil is a huge potential problem for many engines that only have a crankcase capacity of 4 or 5 quarts. So, the recommendation for most people is to have their oil changed every 3K miles to not only avoid engine wear, but also to avoid the problems that could arise from not checking oil levels.
They point me to places such as Blackstone labs, that can give you an analysis of whats in your oil and how your engine is doing in terms of wear. Without taking the engine apart, you can determine a lot by seeing whats in the oil at any time by sending them an oil sample. Some of the major car rental companies use such oil analysis to tell them when its a good time to sell off any particular rental vehicle.
The Blackstone lab analysis has been done by quite a few people on the Audiworld.com forums over the last seven years, as this same topic has been debated on there as well. The results are definitely eye opening for some people who had mistakenly believed the nonsense from the auto makers about the special additives, or the "special" oil that is allegedly put into new vehicles, etc.
On a separate but related note, the one thing that my engineer friends don't all agree upon is this:
How should you drive your new vehicle during the break-in period? Should you be gentle with the driving and keep the revs low, or should you rev it high periodically and hammer on it? There are different schools of thought for both of these, and I see some validity to both arguments. What I haven't see are comparison engine tear downs after be driven under those two different conditions for those first 1K+ miles.
My response post is not meant to criticize anybody or their oil change methodology. The above info is merely meant to add to the discussion, and give some insight on the topic.
For me, I change my oil during the break-in period, at 700-1000 miles, and I'll change it again at 3000 miles, and then every 3000 thereafter. I also change the transmission fluid and differential fluids after the first 5K miles, and then more often than the mfr's recommendations. I've done that for over 20 years, and I've put 150,000 miles on a number of vehicles using that approach, and even after that many miles none of those engines were burning oil, and I haven't had any tranny failures, etc, despite those vehicles being driven very hard.