if you had it heat high enough it very likely worped your heads and or damaged the head gasket
If it is leaking from a bad head gasket, it is because of a bad head gasket. If it is leaking from a bad intake manifold gasket, it is because of a bad intake manifold gasket.
if its a 3.1v6,then it is a intake manifold gasket problem, the 3.1 a noted for this!! check to see if gasket is bad.
If really on top then probably the intake manifold gasket ,study it closely .
I just had the intake manifold gasket replaced on my 96 century yesterday and my bill was $450 but my gasket was only leaking out. If its leaking into your oil it may be more.
Most likely the intake manifold gasket is leaking.
You either need a head gasket or intake gasket.
most time it is caused by intake gasket failure
Intake manifold is leaking, or worse, cracked cylinder most likely tho, the intake manifold.
leaking gasket try tightening the bolts a little might help
maybe it's not the intake gasket. Maybe it is the intake manifold itself, they were bad about cracking. Also there is a coolant line that runs under the manifold that likes to leak as well. It's more likely to be a ruptured/cracked intake manifold as this is a common problem on these cars. See detailed info on the problem (and how to fix it) here: http://www.autoclinix.com/Manifold.htm
Look for leaking antifreeze/coolant. Usually you can see it leaking around the top of the engine. Also check your oil, if it looks milky it has been mixed with coolant and you intake manifold gasket is leaking. That being said, it is possible to have a leaking intake gasket with no sign of leaking antifreeze. Most intake manifold issues/leaks are coupled with poor engine performance and even sputtering and lack of power.
Intake manifold gasket leak....big problem with that engine. Sorry for the bad news.