How do you know its time to change transmission fluid?

We had some problems with Dodge pickups with standard transmissions shifting in cold weather back in the 80's and Chrysler told us to drain the trans fluid and fill them with automatic transmission fluid. Automatic trans fluid!! But it worked!

You're not giving us a lot to go on.

If it is an automatic transmission have it serviced according to the maintenance schedule. Low fluid in an automatic transmission does not cause gears to hang in, it causes them to slip. If it is an automatic transmission, you are probably describing hanging in gears too long while upshifting, so the engine revs higher than it should. That is almost always caused by an engine problem - you are having to press the accelerator down too much, which signals not to upshift yet. A mechanic can find and fix the cause of your poor engine performance when cold.

If it is a manual transmission, it is low on lube.

Your gears are difficult to shift?

In what?

Automatic transmissions can be difficult when ATF is low or there are mechanical defects. You can change the fluid and filter to clean it up. A little old fluid remains in the torque convertor, but most sediment gets caught in the filter so don't worry about what the torque convertor retains. If the transmission is slipping or shifting slow it needs to be mechanically serviced- surgery, not medication. First do on OBD2 code check to see what codes may be store and those will tell what is malfunctioning.

Your transmission gear oil is old. Drain it out and put the recommended weight back in but spend a couple bucks more and get the synthetic. Synthetic doesn't thicken up when it's cold so it will solve your cold difficulty issue.

Fluid change intervals are listed in the owner's manual. However, if you've got a manual transmission (which is what it sounds like from your description) then it's possible your car has no recommended transmission fluid change interval. In many cars with manual transmissions, the manufacturer doesn't call for a transmission fluid change/fill until or unless the transmission develops a leak or has to be opened up for other major work. At that point you'd fix the problem and change out the fluid - otherwise there's really no reason to change it because the fluid lives a very easy life compared to other fluids in a vehicle.

In other words, the fluid itself is likely not the problem. What's more likely is that you have a mechanical issue that's causing difficulty shifting gears. The issue could be transmission related or it could be clutch related. Or both. There's really no good way for me to guess the cause from internet distance - you might consider having someone take a look in person to give you direction on the cause, but I wouldn't go in expecting a simple fluid change to solve the problem. Good luck.

READ the owner's manual that came with the car. It will tell you when you should change the transmission fluid AND filter.

No owner's manual? BUY ONE and THEN read it.

When you either hit the mileage recommendation to do so or the fluid is becoming dark or burnt.

If it smells burnt or is very dark it's time to change it. Maybe too late.

The change interval is in your owners manual. The shift linkage bushing could also need attention (happened to me on a Honda and a Ford).