I am changing the fluid and filter on my 98 LeSabre Trans and the instructions state that if I have a reusable gasket to reuse it if in good condition or use the new flat neoprene that comes with the filter kit.

Should I reused the gasket (first time the trans pan has been off) that was on the pan or should I use the new flat neoprene gasket that came with the kit? Car has 194K miles.

IF its never been serviced in 194k it could start to slip after you change it best to leave it alone.
Unless you have transmissions problems going on now.

If your having some problems its possible a change could even make it worse.
Its like Clint East Eastwood saying are you feeling lucky I for got how many bullets I fired at you do I have one left or not?

I had a guy that does transmissions for a living and has been doing it for many years; and he stated that was not true. In fact, he stated that it is long overdue as trans fluid breaks down over many years use due to heat, etc. If the clutches are worn to the point of slipping, they are going to slip regardless if the fluid is changed or not. Someone stated something about clutch material in the fluid could possibly be helping the clutch discs and by removing it with a fluid change could possibly cause slippage; however, those particles are not attached to the disc so they will just slide along.

What I want to know is if I am more likely to have leakage at the pan reusing the reusable gasket installed by GM or switching over to the new flat neoprene gasket that came with the kit.?

The factory one is metal reinforced so better if its in good shape and not torn
I guess you feel lucky I think every one here would say I was correct but its your car.

Actually, if you search some will say yes and some will say no. If those saying yes, it will cause problems, are transmission guys then I might think there is something to it. I have had a transmission guy, a guy that does it for a living, say there is nothing to that claim.

However, let'* discuss it. Why would changing the fluid; fluid that is old and has broken down to some extend, cause problems? What is the reasoning behind the claim?

Some thing to do with the clutch material is in the old fluid keeps the clutches from slipping.
Ones new fluid is added with out the clutch material being more slippery it can cause the transmission to slip.
Of course depends on the condition of the transmission.
The old rule was if it never was changed at like every 30k or so and its over 100k
was to not change the fluid.
Take it to your transmission friend and have him guarantee it wont start to slip after he services it.
If he is so sure I wonder if he will actual back up his claim and rebuild it free if it starts to slip after he services it.

It is clutch material bonded to the disc that locks the disc pack; loose material would just slide on the surface. If material in the fluid could keep worn discs from slipping, I am sure someone would make an additive with clutch particles in it and market it. Let'* use science here. The clutches in the servos wear because just before they lock up, there is slippage. Eventually with enough miles enough of the clutch materials wear away that the servos can not fully lock up the clutches and plates. If the clutches are worn, they are worn and putting clutch material in the fluid is not going to change that.

Yes its called B&M TRICK SHIFT about $10 a quart at auto part stores

Show me where this fluid has clutch material particles in it.

Let'* put an end to this myth. Changing the fluid in your transmission will not cause it to start slipping. In fact, you don't want clutch material floating around in your fluid, it will only cause problems. Hydraulic fluids can break down over long periods of time due to heat and oxidation. If your transmission has not been service with over 40K miles, then do it as preventative maintenance. All auto transmissions will eventual fail with enough miles; that is just the nature as the clutches will eventually wear out; just like the clutch in a manual shift will have to be replaced at some time in the life of the car if it is driven long enough.

Well its all I can say I have seen it used many times to fix a slipping transmission and they all said same thing {Man I just changed the fluid and filter and it stared to slip }
I don't know what'* in the stuff most likely clutch material or other as you said