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I'm sure those that engineered it know, but without any precise means of checking it and no cooler, I reckon they didn't think it would get hot enough to hurt anything.
That said, as someone who works with temperatures and heat transfer all day at work, I can't help but be curious as well.
I dont think there is a optimal manual trans temp long as the lube isnt cooking and coking. A optimal weight oil perhaps for a given climate. If you ran 80/90 in a siberian winter Im sure you'd be having plenty of fun shifting. Run a good synthetic thats dosent turn to mud in sub zero temps and you'd be good to go. Big rigs run diff and trans temp gauges,you could be the first here to see what you get adding them.
I don't know that I'd bother replacing a good synthetic 10W-30 with MT-90. MT-90 is good stuff but the synthetic 10W-30 has been found to be very good too. At least there are no reported issues with it.The 10/30 is a great way to flush out the syncros if you suspect a harmful live had been used. Run it for a bit then install the MT90.
So long as you're running the right transmission lubricant in your AX-15 there's no need to worry about its temperature. Technically the lube is supposed to be a GL-4 75W-90 but that's next to impossible to find in auto parts stores. Redline MT-90 is a GL-4 75W-90 (synthetic too) but it's pretty much only available online. A GL4 75W-90 is so hard to find that Aisin, the manufacturer, came out with an alternative suitable lubricant which is a synthetic 10W-30 engine oil. No worries about that viscosity seeming to be grossly different from 75W-90, it's essentially the same physical viscosity. Gear lubes and engine oils are measured on two different viscosity scales.
There is one more acceptable lube. SOME GL-5 75W-90 gear lubes are acceptable, but only the ones that say something like "Safe for yellow metals" which means it won't harm the brass synchronizers in a manual transmission. Not many GL-5 gear lubes are safe for yellow metals but it doesn't hurt to look at the back label on newer synthetic GL-5 75W-90 bottles to see if it's safe for yellow metals which means it'd be fine in the AX-5 or AX-15.
I can't imagine Chrysler adding 10W-30 as an approved lubricant solely on their own without having also consulted with the manufacturer, Aisin.Aisin didn’t update their recommendation, Chrysler suggested the 10w30 because they didn’t have a 75w-90 GL4 to sell people. Aisin would still advise to run 75w-90 GL4.
I can't imagine Chrysler adding 10W-30 as an approved lubricant solely on their own without having also consulted with the manufacturer, Aisin.