the service manual suggests ATF only in cold climates , 75-90 is recommended in warm climates. do not use anything rated GL-5 it is too slippery & the syncros will not grab & the gears will grind . Redline MTL [manual trans lube is good too]
They sell it at just about all the automotive stores.
I know Brewer's Performance sells/recommends Pennzoil 80W-90 weight GL-4 gear oil and since the Pennzoil website indicates that synchromesh is the only non-GL5 MT gear oil they make, I assume it's the same stuff. Anyhow, it seems to work just fine for me.