2003 Tacoma: 80W-90 for Diff AND Transmission?

could well be. however, there are sulfur compounds routinely added to extreme load worm drive gearboxes, and they often have copper-based grown gears. and some level of sulfur ep additives are routine in synchro vehicle transmissions that use copper-based cone clutches.

i guess we're coming down to the specifics of the /type/ of sulfur compound, their stability, the specifics of the alloy, and the specifics of their environment, like temperature, exposure to oxygen, etc.

best policy is that, unless you have specific knowledge, just stick to manufacturer spec. the old mechanics joke is about the customer complaining about a $200 bill for "you just hit it with a hammer - that's not worth $200!" and the mechanics response of "hammer blow - $1, knowing where to hit, $199.". you might be able to buy alternative oils cheaper, but knowing /which/ one to buy is what you're paying for.

that would bother me. "stiff shifting" is caused by the synchro clutches not clutching. that may be fine short term, i wouldn't want that situation to persist if it was a vehicle i was trying to make last any length of time.

I think a lot of it has to do with the specifics of the alloy, yeah.

The problem is when you wind up with a manufacturer spec for a product that hasn't existed for a few decades. Then translating that to a modern equivalent can become an issue.

--scott