In my experience of working for just about each and every Class 8 truck manufacturer except Paccar, oil samples prove to be very informational.
The tests are ussually submitted by two types of people / customers. The first is the fleet customer and the second is the used truck department.
The fleet is trying to check out where they are (lifetime) with an engine. Example, a fleet has a furniture customer that wants new trucks in the summer, but the old trucks are questionable in the area of whether they will make it untill then or not. Then the fleet wil make the judgement call to overhaul the engine, leave it be, or sell and buy new trucks early. The time that the samople is taken is ussually dictated by a financial or mileage prospective.
The Used Truck departments use the samples to keep their profits up, customer satisfaction up and costs down. This keeps customers from claiming that they were sold a lemon etc. Why did the engine fail? Etc. It gives the Used truck department some ammo, b/c those trucks are rarely sold as is. There is always some work to be done on the truck before it is delivered. Protection is a must.
I have never seen where someone samples thier oil when ever they have thier oil changed. I would feel that this is a mutre point as the sample company is comparing your engines oil to many diffrent engine records vs you comparing to last oil change. Parts usually indicate wear (metallic deposits) way before they fail.
As far as extended oil chage intervals, I am not a big fan of them. The newer class 8 engines ISX, ISM, C-15, etc. all have filter systems from the engine OEM that allow for extended service intervals. In differentials and manual transmissions they are almost doubling the miles to around 250,000 - 300,000 mile if synthetic is used. 150,000 if mineral oil is used. Engines for me are different b/c there is such a likley hood that your oil can be contaminated easily. Intake air leak, oil cap fell off and stuff like that. Waht makes me question extended rain intervals the most is that when you are talking light duty trucks like the F250 / 350 the owners have ussually modded thier trucks and are pushing oil towards its limits in a normal situation. Then they want to extend the abuse. As emmisions dates go by (2007, 2010) the oil in the crankcase will have to absorb more soot. The most that should be is 2% - 3%. They can not afford to have it dmage the high dollar exhaust treatment filters. PC-10 oils will do the job but engine manufacturers are requesting an oil that will hold more soot.
When ultra low sulfur diesel gets mandatred across the contry you will see extended drain intervals start to become more legitimate. The lower sulfur will help less sulfuric acid form in the crankcase.
When CJ-4 oil is introduced to the market it will be interesting. The CJ-4 is compatable back to approx 1994 engines. Do to the material that the seales were made of and the lax tolerances vs. today you will have oil leaks all over th pace on engines. Put the CH-4 back in it and it will be fine. Just one of the characterisics of the new oil.
Sorry for rambling on its just that one subject lead to another and to another.