Well, here's what I did on both the Suburban and the 560. I switched to Castrol Syntech semi-synthetic with no problems at all. I have logged almost 10K miles on the Suburban and over 4K on the 560 with no leaks. The 4.5 still runs full dino oil.
I was worried about possible leaks when I did the Suburban but those worries were gone by the time I got to the first oil change. My plan for the 560 is to try to switch to full synthetic at the next oil change.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.