Take it to a mechanic you know and trust. Depending on how "good" the mechanic is, they will charge you between $350 and $800 to do the job. Make sure the mechanic re-machines the intake. This may take an extra day, and they may have to send it somewhere else to get done. If all he/she does is take the intake off, replace the gasket, and put the intake back on, that's not a real job. It has to be machined.
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.
Replace the gasket.
Yes it is important. You can loose gas mileage and do damage to the engine over time.
Plan on at least 6 hrs.
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Head gaskets do not go bad on these. You need to replace the plastic upper intake manifold as it is leaking coolant into the combustion chamber. Common problem for this engine.
The head has to be removed on the 1993 Roadmaster to get to the gasket. You will need to remove the intake, exhaust manifold, and valve cover then unbolt the head to remove it. Check the head for warpage while it is removed.