Notice they say "MAY be used" and not "recommended to use". Also they mention many different weights of oils up to 50w in the pamphlet. They didn't say to use 30W in transmissions. I stand by all my statements. 80/90w is not engine oil ! and engine oil is not gear oil. They say in the TM's you can use gasoline in a deuce too. Are you telling me that, that is the "recommended" fuel for a deuce ? I don't think so. Just try and use gasoline as a steady diet for your deuce and see what happens. Same with this non-detergent engine oil. Use it as a steady diet and see what happens.
I'm not going to entertain another senseless argument over the exact labeling of oil products when over and over lube manufacturers state that non detergent engine oil is interchangeable with gl-1 gear lube. Especially when the msds is almost exactly the same if not exactly the same depending on manufacturer. They both lack any notable additive package because they are both just base oil.
I'm not trying to say that 40w is superior to whatever lube you choose to use. But to say you can't use it or that 40w, 30w, or whatever won't work just fine is false. Period. Especially with hundreds of transmissions way more advanced and complex than a spicer that specifically state to use engine oil.
Never mind the hundreds of deuce owners who run engine oil in the transmission that encounter no problems with it at all.
Do I recommend 40w as an acceptable transmission lube based on personal experience and useage? Yes I do. Is it best for every truck in every climate? Probably not. Use what works best for you.
Heck the most up to date tm states to use gl-5 gear lube. My truck didn't shift worth a darn with gl-5. With 40w it shifts perfect. Is that the case with every truck? No.
O and gl-1 isn't 80w-90. I never said that.