A little background...
~12,000 miles on a trans rebuilt by O.C.
Tangerine shift linkage
Tangerine firewall shift bearing
All bushings replaced
80w90 dino oil - changed about 8 or 9 months ago.

Shifting is perfect when it's warm outside; nice, easy, fluid. I don't have to think about what I'm doing. Gear changes are effortless.
When the temps get cold (the high today was 45), shifting gets pretty difficult. Imagine trying to slice ice cold butter with a cold knife...but not the blade part of the knife, the flat side. It takes a good bit of effort to get from gear to gear, which makes my shifts really slow. So much so that I'm needing to double clutch upshifts because the shift lever literally isn't able to be moved quickly enough. And it doesn't get any better even after I've been driving for half an hour and the car is fully warmed up.

Anyone else have a similar issue? Should I try a different gear oil?

I've had other cars where the shifting gets a little tough when it gets cold out, but none that were anywhere near this bad.

Last winter I had several cases with my then tail shift where I could not, for the life of me, get the car into reverse when it was really cold. I'm talking generally bellow freezing ambient temp. I would think I was in reverse, but it actually went into second. After a few minutes of engine warming up sitting there in garage (or in parking space at work), I'd eventually be able to, but then shifting was generally a little "sluggish" thereafter until the car warmed up. I never figured it out, and nobody offered any plausible answers. Hopefully my side-shift won't do the same thing this winter. I ran/run 80w90 dino gear oil too.

At least it sounds like yours went away somewhat once your car warmed up. Mine is like trying to row an oar through caramel, and it stays that way for as long as I'm driving. It's as though the entire assembly has contracted in the cold and doesn't want to move. Or like the gear oil never warms up at all, but I don't know how that could be.