Gasket that goes between head and valve cover to prevent leaks
When it leaks.
Oil/antifreeze mix is usually caused by head gasket leaks or valve cover gasket leaks.
NO. The valve cover gasket seals the valve cover from leaks. The valve cover covers the valve train and it's components and it bolted to the top of the head. These components sit atop the head. The head is bolted directly to the engine block with another gasket. Water and oil flow through the head.
replace the valve cover gasket, easy fix
The valve cover gasket is damaged. Needs to be replace.
Check the valve cover gasket, if not that you may have a warped head.
Valve cover gasket needs replacing
Yes, if it leaks oil through and on to the spark plugs, it will foul them and cause a missfire.
rocker cover gasket or valve inspection gasket leaking
In my experience valve cover gaskets typically don't break until you remove the valve cover. Some (especially older style cork gaskets) tend to harden over time. Once that happens the combination of heating/cooling that expands and contracts the valve cover can cause leaks to develop. When the cover is removed to replace the old leaking part, some of the gasket might stick to the cylinder head breaking the gasket. Cheers
Tighten each one uniformly just until they stop, any further and you squeeze out the gasket, any less and it leaks.