ZF oil change

[birdshill123]

I am considering changing the oil in my ZF tranny to a synthetic.
According to the ZF web site this will allow me to go 2 years and up to
75,000 km depending on the brand of oil. Transynd and ZF oil have the
longest change interval. I also noticed that with Dexron the interval
is every year. However I would think that the synthetic would handle
the heat from the "retarder" better than dino oil. Then I opened up my
Blue Box and it shows the capacity as 47 quarts!! I realize that not
all the oil will drain but that is still a lot of oil. Questions: Have
any of you changed to a synthetic? Does ZF have a special procedure for
this ( Allison does). Do you think it is worth the expense ( about
$5.00 a quart or more)? Any tips on changing the oil? BTW : For any
Amway, oops I mean Amsoil lovers, this oil is not on the approved list.

Bruce 1988 FC 35