I just started using it in my 1986 420SEL which has 313,000km. I was using quaker state before and figured it is the same oil, just with some additional additives to condition seals and such, so I put it in when I changed the oil last week. I didn't find it cost much more than regular. I figure it will take some time to figure out if it is helping at all. I'll let you know over the next few months.
1995 E420 - retired
1986 420SEL - retired