I was told at work that you're supposed to use synthetic oil with turbocharged cars, but I haven't bothered with it in my capri, since I change the oil often and rarely drive it.
There is no difference between conventional and synthetic blend, ALL oils currently sold, excluding straight weight SAE 30, 40, 50 are synthetic blend oils.
Full synthetic on the other hand contains a host of fun additives in it's base, depending on the brand, which can clean up your engine and defeat particles before they even reach the filter, which is a big benefit in a car with the engine oil being used in a 15,000 rpm turbine bearings.
Synthetic's carry a high cost though, fram recently started branding a full synthetic oil for ~$5 per quart, which is produced by Conoco-Phillips, used in 76, Kendall GT, and even Motorcraft. When I change my car I plan to use this, since it's affordable, and since we sell it at work I get a nice discount.