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This is an uninformed personal opinion, but I don't think oil loses its lubricating properties over time as much as it is that the oil becomes contaminated.
The size of the oil reservoir could also be a factor, i.e. 9+ quarts of the stuff, so the contaminants get dispersed throughout a larger quantity of oil. Plus, maybe the filtration systems are better today, also leading to longer change intervals.
Nevertheless, the oil change interval on my Boxster was about 6k the first go-round and will probably be about the same this next time. The one year interval will arrive long before the 15k miles.
The '81 911SC I had before the Box usually had the oil changed at a one year interval, and the mileage varied between a low of about 800 miles (!) to a high of maybe 7,000, and the Porsche-trained wrench I took it to grumped about that one.
The night after I bought that SC I was looking through the maintenance documents, and ran across one oil change bill with the notation in large block letters "MUST CHECK OIL REGULARLY. OIL 7 QUARTS LOW!" An oil change on that car took about 11 quarts, plus maybe another couple of quarts in the oil cooler and the lines running up to the loop cooler in the front fender, so the total system had maybe 13 qts. or so.
I suffered a serious case of buyer's remorse after seeing that bill, plus another one that noted a 6qt low condition at another change. However, I owned the car for 7 years and the engine was none the worse for it's low-oil experience. It is hard to hurt those 3-liter motors, and there is a benefit to a large oil capacity and a dry sump system.
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