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Maybe a little outside the box thinking... (easy when it's someone else's tranny).
Honda requires their special Honda MTF in their manual trannies... but if it isn't handy, they say just use 10W30 engine oil. I've seen reports of people using 10W30 in Honda trannies for 200k+ miles with no problem.
Why?
Well, the viscosities aren't too different between engine oils and gear lubes, but it's hard to compare directly at cold temps. The applicable standards are set up slightly different, SAE J300 covers engine oils, and SAE J306 is for axle and manual transmission lubes. Both have specs for viscosity at 100°C and there is a lot of overlap. A 90 grade gear lube will be between 13.5 and 18.5 cSt at 100°C. A 40 grade engine will be between 12.5 and 16.3 cSt at 100°C.
The specs for cold temps are not directly comparable. Based on the Honda MTF (which is thinner than most gear lubes) being comparable to 10W30, I suspect 15W40 to be pretty close to 75W90.
So...
maybe try a 0W-40 or 5W-40 engine oil to get thinner oil at cool temps but match the viscosity of the 75W-90 at high temps.
Granted, there is more to lubricants than just viscosity.
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