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.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
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