Been running gl5 in my series boxes/diffs for a decade or so, not blown up yet!
Basically so long as it roughly the right viscocity it will work.

Guy on the s2 forum stuck some gl4 and 5 into two jars and stuck some yellow gbox bits in them, after a long time he could see no difference in the jars.
I dont know if there is any yellow metal in a series anyway, I only found out about it when I had a turbodaily
 
I know years ago big power presses that had phosphabronze bushes and bearings needed special lubrication. A factory I worked at ran a couple of their 100 ton presses on a non recommended lube and the bearings failed within a year.

Col
 
Many old cars and bikes recommend GL4 rather than GL5 because of yellow metals within the axles and boxes.
While I am unsure just how much of a difference it actually makes I am not sure why anyone would use GL-5 in an old vehicle when GL-4 is readily available?
I just keep stock in the garage for when I need it.
 

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