ATF levels wierd

[minsk]

so i took a reading of my atf levels with the car running and at temp.
nothing shows up on the atf dip stick.

my understanding after reading articles on this forum is that it is supposed to be a few mm under the lowest reading.

so im freaked out...it is slipping.
also is it ok to top off with modern atf fluids...
iread on here to get castrol (or other brands)dex III no one in any local flac has that.

i have a 1982 300d with 150k on it