Transmission fluid question

I know this topic has been beaten to death but as much as I've read and searched I still cant seem to come up with a definite answer that makes me comfortable. I'm doing a lot of work to the newly acquired car bc as soon as I got it home I went to work really inspecting it and the previous owner definitely did not maintain it nearly the way I would have so before I start driving it I'm going to go ahead and do quite a bit of maintenance. I have about 12 quarts of pentosin ATF1 left over from my 325 (zf trans) and of course this one has the GM. I just dropped the pan and the dirty fluid at one time was definitely brown like the fluid in the zf. I've done alot of research and pentosin claims that ATF1 is also compatible with the GM trans and I'm having a hard time deciding whether to use the maxlife I bought or to use the pentosin. I also read that while you can use a dextron fluid in it, you cannot mix the dextron with the original Texaco fluid. So I'm leaning toward using the pentosin bc it's the same color as the original texaco. Does anyone know if there will be any issue with the pentosin mixing with original fluid? I'm planning on doing another drain and fill soon after anyways so if anyone has heard the pentosin can't be used in this trans please chime in and let me know. It drives me insane that it's so hard to get a definite answer on what fluid to use since the original is discontinued!!

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Transmission fluid question