"As for the Porsche recommended 15k mi oil change - whats wrong with that? Doesn't the manufacturer know their cars? It seems to me that this is a design specification that the engineering team worked to. Also this is a closed system with a dry sump. Also we are using synthetic (non-decomposing) oil. It makes sense to me. I don't mean to be argumentative, I am just wondering whats the rationale for these frequent changes."
First of all, the rec is 15K or one year, whichever comes first. Many Boxes don't get this many miles in a year, so the drain interval in miles is usually less than 15K.
Secondly, Porsche recommends an oil filter every OTHER change, which is exactly opposite to what most oil engineers would tell you. In fact, where truck fleets use synthetic oil and change at 25K miles, many actually change the filter at 12.5K miles, as recomended by AMSOIL.
Lastly, keep in mind that Porsche is at cross purposes to some degree here. They get a lot of flack for service costs as it is and so scheduling them out to 15K gets them off the hook to a certain degree. So does the fact that if your motor craters after warranty, well, that is your problem, not theirs.
All in all, I tell my customers to go every 7500 miles with filter included. When you do the oil change the way I do, the cost is pretty cheap, esp as compared to say a $12K engine replacement.
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