The 4.2 and especially 3.8 M116 are somewhat slower by today's standards, but they are very easy to work on and will run forever with very low maintenance costs. Just make sure you do your guides every 100K and keep an eye on the plastic oil tube connectors, and you'll be good to go.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280