Trans Fluid Question

[I posted this last week but it seems to have disappeared. Sorry if it's a dupe.]
How exactly do I know if/when I should add transmission fluid? The dipstick
just has two lines, so I assume as long as the red falls somewhere between the lines it's alright. Am I right on this? I don't want to overfill it.
Second question: What kind of trans fluid should I use? I noticed some of them have different substances, but which ones are okay to use in a 300CE? I don't have a service manual to ask. Thanks!
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yes, with the car in park and idleling the trans fluid should be between the marks on the dipstick. I've accidentally overfilled slightly before and experienced erratic operation, such as not selecting the gear forward or reverse and or stalling, till I drained it back out.

a
dipstick
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