I am also going to be replacing the rear main seal in my 2.1.  I have to pull it apart to replace the clutch I killed push starting the van earlier this winter.  I figure I might as well replace the rear seal since the labor is mostly done at that point.  I was reading on GoWesty's site that they recommend only using the VW OEM seal.  Is there any thruth to this?  Has anyone had bad experiences using other than VW seals?