I don't know anything about the e300. I have
however read a few gm manuals from the 60's
+70's. The advice given on changing rear-end fluid
was not to. If the rear-end presents a problem
requiring it to be drained so it can be examined/
taken apart then change the fluid.
I don't agree with this. Today when I changed
the (probably) 34 year old fluid in a 1970 chevy
c-10 it was dark brown and seemed to come out
in clumps, though there was no solid material.
It seemed to have a consistency more like motor oil.
In may have been just a buildup of moisture but
in any case I'm sure provided less than ideal
lubrication. Having experienced rear-end problems,
I'm sure this was good pm. If you are shopping for
a 14mm hex let me know. I sold the 82' sd and
don't really need it.