The 617 I5 engines on the 1970's-1985 300SD's are bullet proof and will run a long long time with no problems as long as you perform their normal required maintenance. The 603 I6 Engine on the 300SDL is also very well built and will also last an extremely long time but you have to pay extra attention to maintaining its cooling system as an overheat can cause the aluminum head to crack...which is a $2k repair....the SDLs are a little more costly to maintain than a 300SD but not by much.

The 350's require a lot more maintence, they suffer from the rod-bending issue, and they are also quite a bit more rare/expensive to obtain. Also they have more gadetry and computer items on board that can break. These are definatley the least-reliable of all the diesels, MB got it right again in 1995 onward with the 606 I6 diesels and later the turbo model....

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)