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Today's oils have more additives in them than just base oil. TBN is a measure of how well it can neutralize acids. Some combustion byproducts can be acidic. If those get into the oil, they need to get neutralized or they can start to eat the metals. I'm surprised that the value dropped that low after only 1200-miles. But, as the description I linked, it can drop fast, then plateau. There could be other bits in there that help. So, basically, I don't know. Another test at say 3000-miles would be instructive, but the OP may have changed the oil at that point, and it's mute. I haven't tested in a long time. But, I've seen higher TBM values after 12K-miles than that 1200-miles. SO, I'm somewhat wondering if it was accurate. It would be interesting to know what the TBN value of virgin (unused) oil was. My expectation is that BMW knows their engines, and it may be that the M-versions call for an oil change that early, and if so, sounds like a good idea! That may not apply for their 'normal' engines, assuming people abide by the break-in suggestions FWIW, I've tested some oils at 12k+ miles, and they were still in fine shape, TBN good, wear metals low, viscosity within range. BMW engines tend to use more oil than many others, and that helps. Synthetic oils can handle heat better than conventional ones, and, if you don't use up all of the additives, they can last a very long time.
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