Apparently the W126 chassis axles were some of the last ones that Mercedes specified oil in the boots. Apparently it was similar to gear oil. The oil, from what I’ve read, was like a bath and due to the quantity was not able to fling out of the joint like grease and kept the joint constantly lubricated. I presume that since a tear in the boot meant all fluid leaked out that failures, also, were catastrophic with the oil. Perhaps that, along with improvements in lubrication technology which improved sometime in the mid or early 1990s, caused the switch to a Moly grease.