Oil Change Interval (Definitive)

[Carerra]

I don't mean to poke a bear here.  It is just that I see many differing values quoted around the site for oil change intervals.  

 

I believe the Porsche owners manual says changes should happen at 20,000 miles.  In posts, here and other sites, I have seen recommendations from 2,000 to 15,000.  

 

I have had the occasion to break bread with people who design motor oil additives and motor oils.  They seem like they really know what they are talking about.  I expect that Porsche know what they are talking about as well.  I am familiar with Porsche's leather tanning and finishing standards and they are extremely high,  I expect they care more about their oil.

 

I am not trying to be inflammatory, I just really want to know.  Why (specifically) do people believe that the oil needs to be changed so frequently?  

 

I am not talking about running the car on the race track.  That is obvious.  However, the 911 is know as the best daily driver supercar for a reason. Lots of us drive them like cars.  For those instances, what is the proper change interval?  I am really interested in the experiences that led you to that recommendation.  Also, does it vary by the specific engine.  (Mine is the MA1.01)

[JFP in PA]

It all depends upon how long you expect your engine to live, and how much of the marketing "Kool Aid" that oil producers and car manufacturer's spout you are willing to swallow.  The optimum method to determine when the oil should be changed is by monitoring the used oil's TBN (total base number), which indicates how far gone the additive package is at any point.  When the TBN drops from virgin levels to 50%, it is time to change the oil.  You should also be looking at contaminants that collect in the oil, particularly fuel and water.  Those of us that collect this data, and spend most of our lives inside these engines, will tell you 5-6K miles is about it.  Beyond that point, the oil is falling out of grade (no longer the advertised weights), its film strengths are significantly diminished, and the contaminants start to become corrosive to engine components.