I have an old transmission 090 in a 89 t25. on starting cold, it goes nowhere and seems to wait a bit to let the pressure build up. after a few minutes is is ok but siwtching gears takes a bit longer. If it has gone cold after a drive (say 4-8hours) same problem. whines a bit in park and reverse when cold. i am contemplating a rebuild or a second hand one, but funds not there yet.

i have tried changing the fluid and it is better in the short term. when i add an atf fluid additive is seems better but that too soon stops.

my question is which is better for an old transmission. thick or thin atf?

thicker stuff usually raises temperatures, but its greater viscosity allows it to fill the 'voids' or clearances that are now greater because of the wear and tear, thus being like a liquid gap filler .

The principal works for motors, i can only assume it would be the same for an old auto transmission. Best to wait for a expert opinion though.

The transmission needs to be rebuilt. The seals in the clutch packs have gone hard. So what is happening is the oil is going past the seals and not appling the clutch. Once the transmission warms up abit the seals become soft and
start to do there job again. You can not buy thicker ATF oil. The difference in oils are the additives that the car manufactures use. Most of the time changing the oil makes the problem worse.

There are differences in actual oil viscosities between various types and brands of ATF's. But you need to be using the correct type specified for the transmission.
When you change the oil, you need to check that the screen/filter is clear, and clean out the pan. (You may be able to get 'oil flush' I think to clean the system before draining it, but I don't know the brand.)