I second Mike's idea of gradually switching. Start first with semi and if your engine doesn't show signs of leaks, switch to full. I generally think synthetic oil is for high reving/turbo engines. Where the oil gets really heated up. Personally for a V-8 as large as yours I would just stick with the "change it hot and often" principal, using a good conventional oil such as Castrol GTX. It's really a personal decision, what ever makes you feel comfortable.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section