4ontheFloor,
My limited lubrication experience has been with Standard Oil Co. They made about 6 different gear lubes for transmissions and rear ends. All but one were classified "for hypoid use" and were recommended for every standard transmission made since the dawn of time.
There is no additive in these lubricants that will "eat up a synchro". I would guess if you looked at the spec sheet for the 85w/90 gear lube you are using, somewhere you would find a notation approving it for use in automotive rear ends..hypoid gears. I think this is one of the pesky myths!
Stuart
