I dont know if there is any yellow metal in a series anyway, I only found out about it when I had a turbodailyBeen 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.