I'm pretty sure it's for keeping the blend to run cleaner.
I got this mix from Matt300SD here.
It was 5.95 at Home Depot.

__________________
'87 924S
'81 280SEL

Sold ->

81 300SD -
93 300E w/ 3.2
85 300D-
79 300SD
82 300CD
83 300CD - CA
87 190E 5 spd
87 Porsche 924S

"..I'll take a simple "C" to "G" and feel brand new about it..."