What are the best years for wvo conversion after 1986? I've heard I can go up to about 1999. Is this correct?

Anything MB made before 1986.

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

Some of the models have injection pumps that use only copper crush washers to seal the delivery valves. No rubber "O" rings to get eaten up by the WVO. I do not know a list which models but I have read my 84 300D dose not have the "O" rings in the IP.
I believe the one after 86 have the IP "O" rings and some need a special splined socket in order to change them.

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84 300D, 82 Volvo 244Gl Diesel

Never.

I hear the Chevrolet Chevette Diesel is a good platform for a WVO conversion. The GM 5.7 Diesels are good for WVO too!

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1980 240d
1999 SL500