Porsche: Synthetic Oil, conventional motor, conventional oil

Question
I own an 82 911 with 70K miles, but my question is not just specifically related to this car.   I beileve it is well documented that synthetic oils perform better than conventional motor oil. Generally speaking, I would like to know if it is better to use synthetic oil, and go 10,000 - 15,000 miles (or more) between oil & filter changes, or to use conventional oil, and change the oil & filter every 3-4,000 miles.  The 911 takes 21 quarts of oil, so there is a significant investment here.  Thanks for your opinion.

Answer
You can switch from conventional to synthetic, but once you do, you can never switch back.  On a 20+ year old car, I don't think I would change.  Better to just leave well enough alone I think.  Part of the appeal of owning an older car includes the time you spend maintaining it.  You would lose that time by switching.  By the way, the new 2005 997 911 has it's first recommended oil change at 2 years or 20,000 miles.  It's difficult to believe that one should go so long without changing the oil!