takeuchi final drive oil? sae 90 equivalent?

last time i changed the final drive oil on my tl140, i believe i used 75w-90 gl5 mobil synthetic. when i went to drain this time, the old oil came out dark and very frothy and aerated. not milky or anything like that. just lot of small air bubbles.
the manual callls for sae 90 gl4 oil, which is hard to find. this time i put in valvoline 80w-90 gl5 gear oil. will that be ok?
the one thing i read online is that the newer gl5 gear oils can cause wear on yellow metals like bronze or brass....

takeuchi final drive oil? sae 90 equivalent?

takeuchi final drive oil? sae 90 equivalent?