Sounds like you have the Dex Cool problem GM is getting sued over in two class action lawsuits. One in San Fran and the other St. Louis federal district courts.
The 3800 Series II engine has the very problem you describe. It is not a cracked block. It is fried plenum which was made out of plastic. When it overheats it eventually melts. That's what causes the oil and antifreeze to mix. To fix properly usually entails upper and lower head gasket replacement along with a new plenum, which now is made of metal btw. Not all 3800 II's have or ever get the problem.
The parts don't cost didly. The fix is very labor intense. I have 3 LeSabre's, a 96, 01 and 02. The 96 cost me $1,200.00 to get fixed. Call around, but that price seems to be the going rate. Any mechanic familiar with that engine knows about it. If you let it go, eventually the engine will seize if not fixed.
GM is well aware of the problem and chose to fight it rather than fix it. A Toronto newspaper did an expose' on it a few years ago.
Good luck!
