burning oil smell in vents

2001 mercury sable - for years now, there is a burning oil smell coming through the vents when turning on the vent trying to get outside air. I have looked for the leak and on hot surfaces and for drips and I can't find it. Really appreciate any ideas.
thx
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wrote:

You don't sasy which engine you have, but the Vulcan can have rocker cover and intake manifold gasket leaks that let just a wee bit of oil onto the manifold, where it burns off making a stink.
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