Both from what I've read and my own experience, there's not a big risk with switching to synthetic. Yes, very old gaskets MAY leak a little more than they did, but they will not turn into gushers as big as Spindletop. Synthetics are NOT "incompatible with old seal materials," at least not anything made after 1950.
I switched my 318 to Mobil 1 at *very* high mileage (over 300,000 total, over 150,000 since a rebuild) and have since logged almost another 100,000 on Mobil-1. It leaks a little oil at the front and rear intake valley seals (like Mopar smallblocks are prone to do) but not enough to make me pull the intake or put down a drip pan under it
Synthetics do tend to have very good detergent packages, so if the engine has an unknown history, I'd short-change the oil (and definitely the FILTER) the first couple of times after switching over.
But you also should ask what the benefits of synthetic are. I take advantage of much longer change intervals in two cars (the 318- oil changes every 7000 to 9000 miles, and my wifes 3.5 every 9000 miles) and take advantage of the greater long stability and acid control in another rarely-driven car. If neither of those apply, then it might not be worth the cost since regular hydrocracked oils are really, really good these days. Of course everything with a turbocharger should be running synthetic, as should several modern engines that tend to create a lot of sludge in the oil (Chrysler 2.7, several Toyotas).
Another factor is the fact that our old Mopars don't have roller cams, and do have some helical gear drives (distributor and oil pump). A lot of modern engines like the Chrysler 3.5, the new Hemi, Ford Modular, and the Cadillac Northstar don't have any sliding components at all- the oil pumps are splined right onto the crank, and they have roller cams and roller timing chains or timing belts. As a result, many of the newer spec oils limit the amount of zinc (ZDDP) additives, which are important for high-pressure sliding contact like cams but can slowly degrade oxygen sensors and catalytic convertors. One of the reasons that Mobil-1 "extended life" doesn't have the very latest API approval is because it has a big dose of ZDDP, so I plan to start using it in my old engines with flat-tappet cams.
Yes, I'm a geek.
