The PCV valve on Altimas looks different than what has been common for years on most engines. The PCV valve is located at the drive belt side of the engine under the intake manifold and attached to the breather/separator. Access from above is very difficult; access the valve from underneath the vehicle. There is a short hose that connects to the breather/separator. Remove the breather/separator and the hose. Then you can unscrew the PCV valve from the breather/separator for easy inspection and cleaning of each piece. Also inspect both passages where the separator and hose connect. Here is a picture of the valve after it is removed. The breather/separator is a small aluminum housing that will fit in your hand.
http://www.rockauto.com/ref/SMP/SMPD....html?V277.jpg
Obviously the PCV system is more difficult to service on Altima engines than the more familiar design on so many other engines. A restriction in the PCV system will increase crankcase pressure. This pressure inside your engine will push oil out where ever it can find a place to leak, including into the distributor.