Head gasket

You may find this hard to believe but we can't see it over a computer. If your mechanic said the cylinder head gasket is leaking I have no reason to doubt him. The gasket would have to be replaced. That is a pretty big job but it's common on any car. The repair will take about a day. If your mechanic is conscientious he will also likely recommend a new timing belt if your engine uses one. It will be removed for this repair anyway so this is the perfect time to replace it to prevent trouble in the future.

It is more common for an oil leak to come from a valve cover gasket than the head gasket. Leaking valve cover gaskets are real common and much less costly to replace. That might be what your mechanic said or meant. That repair takes an hour or two.