The above UOA posted on the Renntech.org forums is a good example why Porsche needed to reduce the drain interval from the 2 year or 24,000 mi that they were recommending. Given that Mobil 1 0w40, or most if not all ACEA A3/B3 oils have a starting TBN of 10.0, and now the TBN measured about half, the above owner should be considering changing their oil after 8,500 mi. This is using the rule of thumb that your drain interval should be determined by evaluating the TBN retention and changing when you are at 50% of the starting level. 8,500 mi is still some 3,500 mi less than Porsche's new, reduced 12,000 mi interval.
That is why it is so important to do oil testing on a regular basis, especially with longer drain intervals. I personally use Staveley Services to do my testing. In fact, I have two UOAs from my '91 C4 Cab posted over on my other blog about my 911.
If you would like to share your UOAs, please email me!
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