Half Axle replacement
 

I am about to replace my two from axles and was wondering about the best way to replace any lost transmission fluid. Should I:

a. just add what was lost and will SP-III fluid I purchase be suitable with what is currently in the car or

b. just drain and replace all that is in the car?

I own a 2001 Elantra. And have replaced both front half axles at different times.
Read somewhere that one should not take both axles out at the same time
That doing that will screw up the gear box. I don't know. So, I didn't do that.
Rather to change one at a time. The most difficult part was the stabilizer link.
It just wouldn't come out. Bolt stuck. I cut it off with a grinder.
Then replaced it when putting it back together.
Later, I drained and refilled the ATF with SP-III
Watched for leaks for a few days.