This is common on the V8 M119, but I do not know it the M120 has any plastic guides;
http://www.baxnet.com/merc/oilguides/story.htm
Have a look here to see images of a rebuild M120 and see
if you can find plastic oilers there (I did not);
http://community.forenshop.net/forums/index.php?mforum=W140&showtopic=2090&st=150
Parts drawing;
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=129076&M=120981&GA=722%2E362722%2E621&VIN=%2A&CT=M&cat=15Q&SID=01&SGR=065&SGN=07
The M120 heads are M104 and mirrored M104 heads, so finding it on
a M104 would also answer your question (I think).
Or you could ask this man;-) ; (taken from
http://mbwde.curry.hl-users.com/Mediengalerie/index.htm )
br,
syljua