This topic was intensely debated back in 2013. Agree that in view of the VW scandal this becomes even more interesting...
I think the most relevant debates from 2013 are covered in these two threads (and yes, you need to read through all the pages to find all the relevant info):
http://f10.m5post.com/forums/showthread.php?t=815669
http://f10.m5post.com/forums/showthread.php?t=880707