The crankcase needs to be vented to allow pressure to escape during operation. If your PCV valve or breather are clogged, that pressure will find another way out - usually the dipstick tube. If you have a tight fitting dipstick the pressure can cause a leak by tearing up a gasket to find a way out.
So, a clogged pcv valve can cause an oil leak... but chances are, if something is leaking you need to replace the gasket because it's toast now. Just replacing your pcv valve and breather won't make the leak go away.
If you've got an engine w/excessive blow-by, even if your PCV was fully operational,it can cause excess crankcase pressure. This can cause oil leaks that will go away once the excess crankcase pressure is relieved. The underlying problem still needs to be addressed(worn out motor) but the oil leaks can be lessened or eliminated by venting the excess crankcase pressure to atmosphere. Not a good situation to have either way.
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