JCYHUN,
Thanks for the detailed roadmap to the high side port! Looks like MB went to a great deal of trouble to hide that port. Maybe the same group did the oil cooler line routing--
I agree that either 134 or 12 (the good stuff) should be used, and since you need a license to purchase FRIGC, I'm going to go with a 134 retro. Since I live in a moderate climate, it should work OK.
