I missed this question somehow.
There are a lot of factors that go into the reasoning of our manufacturers suggestion to change our oil at the suggested intervals. However they are mostly based on pressure from big oil to use more. The rest of the reasons are, as you suggested, contamination due to foreign objects being introduced into your oil.
There is a certain amount of fuel that will get into your oil, however it will burn off just from typical heat built up in the engine and the byproduct is ash. This ash will be filtered out by a "GOOD" oil filter as long as it is changed regularly.
I must emphasize that I don't suggest leaving your oil in your car without maintenance. You must change your filters and send a sample of your oil to a reputable company for analysis at regular intervals or you might as well just change it every 3000 miles.
I use extended drain intervals because as an ex-truck driver that used to have to buy oil by the 55 gallon drum, I understand the value of an extra month or two between drains.
Once I got involved with Amsoil ( I am no longer a rep but still believe in the product) I spent a lot of time finding out what goes wrong with oil that could relate to engine failure. There really is not a lot. Metal breakdown, water contamination, viscosity breakdown are about it. If there is metals in your oil beyond the expected manufacturer spec there is a problem. The testing can in fact tell you where to look. Iron is normally a ring issue, excessive copper is normally a bearing issue, zinc is normally a head or valve issue..........
Its amazing what a little information can do for you and much like checkups with your doctor, these samples sent for analysis can warn you far in advance of a failure.
Amsoil or for that matter any full synthetic is very expensive. The testing is not cheap either so there is certainly not a huge advantage to using it over a 15000 mile drain period, however over 1,000,000 miles certainly is. No, I don't expect to make it that far with a gas motor. In theory, it is possible, but not with the use that I give this vehicle. Mostly around town, start and stop driving will never rack up that kind of mileage.
Not brain surgery here. Just common chemestry.
The manufacturers just tend to go the easy way to provide the warranty.
Yes, in fact this will void your warranty. BUT, companies like AMSOIL will stand behind you and even replace your engine for you, IF you follow their suggestions and test your oil.
I now send my oil in to Blackstone labs once per year to be tested but I only drive 12,000 or so miles per year and the car is still new. I change my filter ever three months and only use Wix filters. So far so good for me.