I *believe* oil gets in there from the PCV. I 've had a lot of intakes over the years with a lot of oil in the EGR passage that were good running engines. The factory design gets us agian. The oil will eat the diafram over time and do what you describe.
I usually run a powerplate and do not drill the EGR hole. I also use a PCV catch can. These work for me. And this is only my opinion.:smile: