Yes, that is a very bad idea. The brake fluid will damage the trans seals, and it will start to leak.
Dexron 3 or Dexron 4.
Dexron V or VI
Dexron vi Remove and examine the transmission dip stick. It will tell you what fluid is required.
A 2005 Buick LeSabre automatic transmission requires 7.4 quarts of Dexron III fluid.
Gool ol' Dexron.
dexron III
No you need to use Dexron VI
Dexron III should be a safe bet.
Attached to the rear of the engine is the transmission. There is a filler tube coming out of the transmission and it usually has a red dipstick that you remove to add fluid. Dexron 3 is the fluid to be added.
7 to 7.5L of dexron 3
Dexron III. Any auto store could tell you for sure.
manuel says use ONLY fluid labeled DEXRON-III made for this transaxle