Wrong type (viscosity) lube in transmission is one thing it could be. Hydraulic clutch problem is another, one or the other is about ALL it can be. Does it shift ok, or better, with the engine OFF while cold? Does the clutch pedal feel different after it warms up (more pedal)? If it is difficult to put in first or reverse when cold, try pumping the clutch pedal to see it it helps. Post results for more info.
You have a clutch problem, i have seen this brfore, on the one I fixed it was the slave cylinder! This one would refuse to shift into gear until you pumped the clutch pedal SEVERAL times. It would then be ok the rest of that day until next morning,same thing! You just asseum it's because the engine warms up, actually it is the act of just using the clutch that makes it start working for that time. All of this is just my theroy, i could be completely wrong!! Let us know what you find out.