I would agree that some of the leaks were due to the switch, but I think the biggest factor was the drastic change in weights. If you still would like to use synthetic, try 15W-50 Mobil 1.
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