Simply use your eyes. Buy the full synthetic, if it leaks, stop using it. Go buy some syn-blend oil, if it still leaks, stop using that. You don't mention how many miles your car has or type of driving you primarily do. I believe full synthetic is best for high reving/hot running engines. I use full synthetic in my Passat, which has the 20V turbo engine. Now both of my Benzes have turbos also, but they don't get anywhere near as hot as the one in my Passat. So I use conventional oil in them. What ever you decide, make sure it is the proper weight. European cars like the thick stuff, be it 5W-40, 10W-40, or 15W-50. Just look at your owner's manual, it will tell you what weight to use based on outside temp. "Change it hot and change it often"
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