Except the PCV vapor enters the intake tract downstream of the MAF, so the MAF should not have any fuel or oil residue. The MAF on the 1.8T is in the filter box, that for the 2.8 is separate but still fed only filtered air.
I support what ylwagon said.
It is bad to have any film on the MAF, in a standard B5/5.5 no oil/fuel can be introduced upstream of the MAF.
(Note: If the combi valve remains open, exhaust can flow back into the filter box)