Regarding the tester you purchased. This is right up there with having a third world witch doctor diagnose a serious illness by waving a chicken bone over you, while chanting. If you didn't open it yet, pack it up and get your money back. Sorry, but it's probably less than worthless, as it could convince you to damage your vehicle by providing worthless and incorrect info.
Spent some time on Blackstone's site, read their tutorials, and really get a feel for how complex oil analysis really is. I have used their services in the past, but only for monitoring the "health" of high dollar diesel engines that need $100 in material to change the oil.
Finally, regarding the whole extended oil change scam. We will reach a point, if we haven't already, where most of the semi-knowledgeable used car buyers will know better than to buy used, high end vehicles, since the whole 15-20K change interval is creating ticking time bombs out of a lot of higher mileage vehicles. Bottom line, if you ask the techs. wrenching on these things? the whole extended oil change game is a scam. A good buddy of mine works in a shop that specializes in post warranty service on upper end stuff. For the last five years or so, he has seen the result of the 10-15K change intervals. Changing oil is a process of removing a contaminated lubricant from the vehicle. The oil didn't wear out, although there are many high tech. additives in the oil that are consumed over time. Oil needs to be replaced on a frequent schedule because it turns into an abrasive soup of combustion particulates, dirt from the air used in the combustion process, wear materials from the internal workings and other normal events inside of a very hostile environment. It's pretty common for my bud's shop to get a six to eight year old, $50K German sedan in with a service history of a handful of oil changes, all per the manufacturer's recommendations. The car will have drivability issues, and a quick diagnostic often reveals that the problem lies deep in the motor. This can be damaging wear in extremely complex variable valve trains, worn cams, or similar issues. These cars often get driven into the ground, or traded in, since repairs would exceed the value of the car.
In an ideal world, a car manufacturer wants you to get on the payment plan, and keep you there for life. They want you to think that you are buying a product that is about as close to maintenance free as possible, and they want you to show up the day you make your last payment, to grab your next one. Extended and ignored maintenance are great tools to assure you that you stay on the payment wheel.
Want a car to last for a few hundred thousand miles? First, ignore the manual it comes with. Change the oil every five thousand. Flush the remaining systems (brakes, coolant, transmission) every two years or 30K, whichever comes first. By actually maintaining a vehicle, instead of buying into B.S that the manufacturers want you to believe, you can save tens of thousands over the life of the car, and not hear how the seven oil changes you did in the last 120K miles saved you a few hundred bucks, but it's going to take $7K to repair the damage.