I'm pretty new to this site. I just ordered a test kit from Blackstone Labs. I read the "Gas/Diesel Sampling Procedures" subpage on their website. It says: "Do you want a TBN? This is a test used to measure the amount of active additive left in the oil. It may be useful if you are interested in extending your oil usage. The test costs an extra $10." Can you guys elaborate on these statements? It sounds like they're saying that if I want to go a long time between oil changes a TBN will help me decide if I can get away with it. But isn't that something that the $20 test will tell me? That is, whether I ran the oil too long and its no longer effective -- or I could have run the oil longer and it would still have been effective?