Your C5000 forklift powershift transmission takes Dexron II or Dexron III ATF fluid.
Dexron IV is also a choice, but Dexron IV is a semi-synthetic fluid and there seems to be some disparity in views as to whether you can mix Dexron II and Dexron III oils with Dexron IV oils. Some sites say they're compatible, some say they aren't.
I'd have to say if you went over to Dexron IV, you'd be be best advised to do a full transmission drain and just refill completely with Dexron IV.
Later ATF fluids have a different formula and more additives to counter shudder and harsh gear changes, often found in electronically-controlled automotive transmissions, and to deal with different materials in newer auto transmissions, such as thermoplastics.
The Cat powershift is a relatively simple powershift that doesn't have the problems that affect many modern electronic automotive automatics, so I'd stick with Dexron II or Dexron III.