Just wanted to share a response from ExxonMobil concerning their TBN levels for extended drains. This was a response from them after a question to them via their Facebook page. They bring up a point that I had never considered - if the detergent is "over-based" enough to do the job, a lower starting level is sufficient.
Additionally, there is info online about Mobil 1 being used in NH municipal fleets for 15,000 mile drains per ExxonMobil's recommendation. So, perhaps Mobil 1 is a better long drain oil than we realize and/or are willing to test the limits of?
"Thanks for your message, Mike. We appreciate the feedback. A slight reduction in ash is desirable for advanced engine technology such as gasoline direct injection (GDI) engine and passenger car diesel engines. With GDI engines, higher levels of ash appear to result in a higher occurrence of Low Speed Pre-Ignition. The industry is currently developing tests and specifications to address Low Speed Pre-Ignition. In the future, passenger car engine oil specifications such as ILSAC GF-6 and dexos1 (next generation) are likely to have requirements such as lower ash to address Low Speed Pre-Ignition. As for TBN retention, it is only a single parameter that provides an indication of the used oil condition. A reduction in TBN is an indication that the overbased detergent is doing its job by neutralizing acids that form as a result of combustion. TBN should be used in combination with other used oil parameters such as oxidation, nitration, TAN (Total Acid Number), ICP metals, D4684 MRV viscosity, and D445 kinematic viscosity to determine the overall condition of the used oil. Finally, in our experience in severe-service Las Vegas field testing, Mobil 1 engine oil TBN levels typically do not drop below 2 for vehicles with 15,000 mile oil drain intervals. Furthermore, it is our experience that those oils tend not to drop any lower when we continue to 20,000 mile oil drains. We hope this information was both helpful and informative. Thanks again for reaching out, and for your continued support of Mobil 1."