Check your owner's manual and put in the type of fluid it calls for, and be sure you put in the proper fluid.
I am not sure what kind of filtration system your particular transmission has, some have a metal screen that does not really filter but just blocks debris from getting sucked up into the trans and when you shut the engine off the fluid flows back down and flushes the screen clean again, so does not have to be changed, but some can begin to deteriorate over time if the fluid becomes acidic. So worth dropping the pan to at least inspect its condition. Then there are flat filters with cloth-like filter material, they do not get clogged very easily, they are a very low-pressure system, no real PSI pressure pushing fluid through, more just suction from the pickup tube, they can go a very long time without being changed, But 140K and never had the fluid changed? I would go ahead and change that out as it has had dirtier than normal fluid to deal with. Then some transmissions do have a second actual filter, like a tiny version of what you use for your engine oil. That should for sure be changed out. So I would for sure drop the pan, clean it up, check the magnets for any metal chunks etc. I would do a couple more drain and fills before dropping the pan and changing the filter/filters so they can start out with clean fluid. Hopefully, the clutch material is still in good shape and not depending on dirty fluid for friction, removing all that dirty fluid could cause you to experience some slippage, but that just means your clutch material is pretty much gone already. Stay up on the trans fluid changes in the future.