Trouble going into second when cold.
 

My 81 gsl has trouble getting into second when I first start the car. If I shift quickly from first to second it will grind a tiny bit but it will go in. If I go slow it doesn't grind but it takes a while before it will seat and it has to be under 15 mph and you can't go back to second from third without being under 15. Once the car is warm it all shifts perfectly. I just checked the fluid, it's clean and full. My brother is saying synchros, any other ideas?

are you driving it in Cold weather?..if so there lays the problem.
Most standards are a little funky when they first start off in cold weather.
My FC is almost a 2 handed shift to get to second.

Really that's it? It's worse as it gets colder out but it's practically never below freezing (usually hovers just above in the winter time). Didn't think that would be enough to cause anything... I guess that's why I ask in the new member section. Thanks anyway though.

As you run the car the oil in the transmission heats up and thins down some and it's easier for the synchro rings to cut through the oil film. In general this could be characteristic of wear in the 2nd gear syncho ring but you'd probably be having the same problem in the warmer months too if it was that. As stated there are transmissions in some cars that are just finicky with 2nd when cold.

FD owners commonly have issues with upshifts into 2nd when the car is not warmed up even in warm weather and I've seen similar complaints from S2000 owners.

When I bought my 1991 FC which was new at the time I had them do some work on 2nd gear to try to impove the shifting to not much result.

One thing I forgot to add. You could try changing out your oil in the trans to make sure that you have an oil that is a GL-4 and not a GL-5 unless the FSM specifies that you have to use a GL-5. The synchros will go cut through it better and could offer you some improvement. Make sure if the FSM specifies two oils, a heavier and lighter viscosity oil, like is the case for some generations of the Rx-7, that you use the lighter viscosity one since you are in a cold climate. Any oil that you see will have both a GL and viscosity specification.

OK I'll look into that. By FSM I assume you mean something like full service manual or whatever. You know where I can find one online?

manuals Foxed.ca - Mazda RX-7 Manuals

I have noticed the same situation. Very difficult to shift to second when cold. Takes about 5 minutes to warm up. It happens with 25 year old dino fluid, redline mt-90, and redline mtl, so my experience that it is not fluid related.

Though my fist gen doesn't seem to be bothered by this, i have a Toyota Tacoma that is this way, bought it with 7k and at 187k it's the same way, no worse. When it's cold out below 25 degrees or so, letting it warm up for about 2-3 minutes resolves the issue. I use amsoil in all the drive line components, same in the RX7.