Assuming we're talking the LT95 4 speed manual box here? It should run engine oil - I use whatever I put in the engine, in the box - just easier that way for me. Were you running gear oil? That's a no no in these boxes. Not sure if the thrust washers are a weak point - Bearman would be able to answer that if he sees this thread being the guru on these boxes.
I see no issue with running a 20w/40 synthetic oil.
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