Doesn't sound like a plenum leak. But I may be wrong.

As said, look down the throttle body with a flashlight. If you see oil pooled on the bottom on intake, the plenum is blown. Other signs including pinging with regular octane under medium to heavy throttle, high oil consumption with no external leaks, misfiring from oil fouled plugs, etc.

Lack of power may be an indicator as well, if you've plugged up the cats with oil residue then your engine can't breathe.

If plenum is blown, I recommend in addition to fixing the gasket, that you replace the spark plugs and O2 sensors too. Then determine the health of the cat, and replace if needed.