FWIW, switched both cars over to Mobil's Delvac 1 5W-40 after purchase.
The 617 in the '82 300D had 112K when purchased; no leaks or reported consumption. 30K later, no leaks or consumption between 5K oil changes.
The 602 in the '93 2.5 Turbo had 143K when purchased; no leaks or reported consumption. 25K later and it too remains leak free and does not consume oil between 5K changes.
Summary: synthetic or dino, if the car had pre-exisiting leaks or consumption problems I would look further than the oil selected.
As for oil brand, I'll stick with the majors; ExxonMobil has a demonstrated legacy with synthetic oil technology. Much of the research that went into developing their Mobil 1 line of synthetic motor oils probably helped in the development of Delvac 1.
Just my thoughts on the subject.

__________________
'82 300D Turbo 204K
'10 Toyota Prius 85K
'13 AMG C63 (P30) 23K
'14 Audi Q5 TDI 46K
'16 RAM 2500 CTD 12K

'10 VW Jetta TDI 65K (traded)
'12 Nissan Leaf 34K (returned end of lease)
'15 Audi A6 TDI 25K (traded)
'95 E320 166K (traded)
'93 300D 2.5 Turbo 168K (sold)