I ordered one for the 1983 240D that I recently purchased that had 199K+ on the clock when I got it. My 1981 300TDT is getting close to the first one as well.
The milestones are:
155,000 miles (250,000 km)
312,500 miles (500,000 km)
625,000 miles (1,000,000 km)

I hear that they give your vehicle an added 3 HP when installed.

__________________
Current: '91 300TE 4MATIC 317k and climbing...
Former:
'81 300TD Wagon 168K "Tank"
'83 240D 216K 4spd manual "Da Bear" (aka best car ever)

"Never sweat the petty things...
and never pet the sweaty things."