Long story short / TLDR:
I "fixed" my 5-speed's shifting problems by putting a thinner synthetic gear oil in the transmission.
But maybe the problems were caused by the wrong oil in there to begin with?
The manual gearbox in Mirage #3 has been troublesome since I got it. Even in the summer, 2nd gear was occasionally balky/crunchy. (I figured that's what I got for buying a car that the previous owner admitted was her first manual.)
But as the ambient temperatures dropped this fall, it got significantly worse.
With outside temperatures around freezing and a cold drivetrain, getting second gear was a real problem. (Sometimes 3rd and 1st as well.) Double-clutching didn't solve it. But when the car warmed up, shifting improved.
The plan: mixing a thinner oil
Since it was clearly temperature-related, I started looking at options for thinner oils. Foama suggested mixing some ATF with the usual gear oil to see if that would help. I bought a quart of Royal Purple 100% synthetic MT "Syncrhomax" and a quart of Redline MT-85 (75w85).
When I removed the drain plug to drain the old oil, I was surprised at how thick it was. It seemed like not very much was coming out, and for a moment I actually thought it might be empty! Only after I removed the fill plug did it start to drain properly.
Temperature was around 5 C / 41 F.
Just for fun, I decided to do a simple "pour test" to compare how each of the oils flowed. I poured half a litre of each oil in a marked container with a small hole (~2.5 mm) in the bottom and timed how long it took for the container to drain to empty. I also tested some 5w20 synthetic motor oil for reference.
Time to drain 0.5L @ 5 C Oil type 2 min 25 sec old gear oil 1 min 22 sec Redline MT-85 synthetic gear oil 75w-85 1 min 13 sec 5w20 synthetic motor oil 0 min 56 sec Royal Purple synthetic Synchromax MT oil
Shifting problems fixed!
I ended up going with 1 qt Redline + 0.7 qt Royal Purple, and it's like the car has a different transmission - the shifting problems are all but gone.
Wrong old oil?
The big difference in viscosity between the old oil vs. the Redline raises the question (meaning: foama and inuvik raised it): maybe the old stuff was too heavy to begin with?
Possibility #2: it was non-synthetic and thus too thick when cold?
I could do more research, but I'd rather sit on my butt, speculate and theorize!
I'm just happy the shifting is better.