TH 350, best fluid to use? should I worry about newer fluids?

I have a 1977 Regal, 350 2 barrel (96,000 miles) that very soon will have a quadrajet...anyways...
my Dad called a car show about what to use in a fluid change, and...one person acted like I would be better off to leave the fluid in now.
That sounds, what's the word...crazy? But I don't know too many people that have TH 350's right now, and I am a loon that has a 40 year old daily driver, and I am not really in the position right now to pay for a rebuild if the 350 would go out---only GM automatic I ever had go out was a 69 Impala with a th 350, (and my Mom's 73 Chevy with a th 350, both at around 120,000) but I was a teenager, so I was forever putting the pedal to the metal
So anyways, I'd like to keep this one working aok for at least a few years...
What fluid should I put in? Are some fluids so high in detergents that they could clean out old varnish, and hurt an older transmission?
I had a Lincoln for a decade (fords are a whole other ballgame , what you can and can't use on their transmissions, but the C6 is like the TH 400, nearly bulletproof) so out of the loop on GM Trans fluids...seems like there are many more out there than before...

Oh, and while I am writing an overly long post, I had a shift kit put in my 69 Chevy after the rebuild (was way too firm, even though I asked for a non racing kit, it chirped the tires between 1st and 2nd under full throttle)
pluses/minuses to putting one in a non rebuilt 350????

Thanks!

 

transmission failures after fluid flushes is an old wives tale. i run type f. it's a little thicker and is supposed to firm up the shifts some. ironically, that's likely an old wives tale aswell.

 

The type F has fiction material suspended in it, that's why the old Ford transmissions that required that fluid would burn up very quickly with the wrong fluid in it.

Back in the day they would use the type F in powerglides to like you said to "firm up the shift", not sure if it actually works though?

 

It all depends on whats on the bottom of your pan. If there is a lot of crap at the bottom And has been in the car a long time. do the normal service , BUT go ware no shop goes and remove the transmission lines from the transmisson, stick an empty milk gallon or jug of some sort, a clean one. Use compressed air and blow out the line set, cooler into the jug. after the first blast or two spray a bunch of carb cleaner into the line and blow that through too. Now you have just helped your transmission live longer. And just about every shop uses the cheapest dextron their supplier sells.