I am in the process of changing the glopluggs on my 1992 300sd

i did remove the intake made my job easier but i find out the intake manifold was loaded with a lot of carbon which was too thich in side the manifold. I need to know why it is accumlating there seem like pulling alot of oil through this .any suggestions why it is happening or this is normal.please reply .
my email jmian18907@aol.com or javid.mian@mirant.com.