I have a liberty modified slick shift 833 looking for recommendation on what gear oil to run.

Mobil 1 synthetic 80-90.

I use Dexron ATF. Mine is slick shifted, not Pro shifted. Street/ strip car.

The fluid I drained out was unknown but it did look like ATF.

I raced a 4 speed car for several years in NHRA stock class, one slick shifted tranny until it burred up real bad and then switch to Pro shifted by Nash and Liberty.
Back then in the mid 1970s the Mopar drag race engineers recommended using straight ATF, that work fine
I would make around 40 passes on the car and remove the tranny to replace the clutch disc and pressure plate and look inside the tranny to see if I would have to take it apart to dress up the sliders and engagement lugs or not.
I did that deal for 2 1/2 years and then sold that car and went back to racing automatic cars, my wife liked that a bunch better as I had to have her help me on the tranny R&R with a floor jack in our driveway at night after work
She bought me a Sears tranny jack one year for Christmas

I used to take mine out and it being on my chest then roll over set it on the ground then crawl out from under the car. Very close but I was skinny then. Did that about 7-8 times till every piece was changed out other than case and input. Could afford one part each week lol afterwards I more than likely could have gotten one cheaper out of the junk yard. Dumb and young