Sorry I left that out. . .the car has 125,000 miles on it.
I asked my mechanic about this and he said that if I was ever going to switch to synthetic, this would be the time as the engine is as tightly sealed as it will ever be.
Oil and filter changes were done routinely so hopefully there's not alot of sludge built up. I was warned about the potential leak problems before putting synthetic in my 911 but did it any way. Not a drop.
I think I'll drive about 500 miles (on the new oil & filter put in post valve job) then switch over to Mobil 1. I believe alot of the concern about synthetics is based on problems that occured in the early days. Modern synthetics are supposedly less prone to causing leakage. I sometimes wonder if my valve guides would have held up better if I'd been using synthetic all along.