Regarding the oil... your mileage may vary, but I've never had a problem. Even with a 911 that I used to drive daily, I changed to full synthetic with 30,000 miles on it and never had a problem. Personally, I think the advantages of full synthetic outweigh the possibility of any potential leaks that may (or may not) crop up. Just my opinion though.
Edit: and now a friend owns that same 911SC and it has over 200,000 miles on it... with no significant engine work. It's on clutch number 3 now, with no other problems.
