I have a 2005 Hyundai Elantra. Can I use Castrol Transmax Transmission Fluid?
Castrol says you can, if you're using Hyundai SP-II or SP-III. Since you're out of warranty, I suppose it doesn't matter.
I have successfully used Castrol Transmax Import Multi-vehicle ATF in my 2009 Hyundai. The label should say it is compatible with SP-III. There are several different variations of TransMax ATF. Make sure you buy the right one! I also highly recommend Valvoline MaxLife Synthetic transmission fluid but, again, make sure you get the right one that is compatible with SP-III. You can check the Castrol and Valvoline websites to see which transmission fluid is compatible with your vehicle.
Yes use a different one. But don't mix different fluids at the same time.