The manual calls for 30w, at least in 1990.

Hmmm...my 90 helm for the Trooper/Pickup calls for the multi-weight (and specifies 40wt for hot environs). Not that it really matters in the end. It would not be the first time Isuzu treats the same piece of equipment differently. Does your manual also have the "Don't use 10w40 in a 2.6 or the world will come to an end" warning?

Out of curiosity, what's on the fill plugs of your tranny?

Michael