This is a good question. In 1980, the 240D engine was changed to parallel glow plugs. There were some changes at this point. You need to do two things so you're ready when parts ordering time comes; record the ENGINE serial number located behind the injection pump on the block, and determine if you still have a serial glow plug system. If you do have serial glow plugs, you will see bare, serpentine shaped wires connecting from glow plug to glow plug. If you still have serial glow plugs and you have the serial number, I believe you will be safe in ordering parts for that serial number. If it has been changed to a later engine with parallel glow plugs, you will have a bit of a guess sometimes when ordering parts.
Valve lash spec's don't change, and not much of anything else does. There is a little difference in timing specs between some of these engines, but you will probably never have to worry about that.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in