I'm presuming you have unburnt diesel smoke and not oil smoke.

Unburnt diesel smoke -

Replace the air filter.
Use a fuel additive like Power Service or Red Line.
Make sure the engine has been fully warmed up.
Try for a source of fresher fuel, like a truck stop.

Oil smoke -

Change the oil and filter just before the test.
Use STP Oil Treatment and 15W-40 conventional oil.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles