you have a blown head gasket water is getting in to the engine oil
No, only a blown head gasket or cracked head will cause coolant to mix with the oil. A leaking valve cover gasket will only cause oil to leak onto the engine block externally.
If the leak is bad enough i believe it can but i would check your valve cover gasket first.
you mean a valve cover gasket i think
Rocker gasket? If you are referring to a valve cover gasket, no it will do no damage as long as you keep the oil level full.
Bad engine seal? Bad oil filter? Leaking valve cover gasket?
If by "under" you mean below, the intake gasket may be blown out. If you mean inside the valve cover, you may have cracked block or head or a blown head gasket.
it could be a number of things including..timing,thermostat,fan clutch,blockage in radiator,or blown valve cover gasket seals or worse blown head gasket
The head gasket seals the head to the engine block. Coolant and oil flow through the head. The gasket prevents then from mixing together. The valve cover gasket only prevents oil from splashing onto the engine and exposing the valve train and moving parts to the outside of the engine. No coolant makes contact with the valve cover gasket.
The most common place for oil leaks is the sump/crankcase cover gasket. A torn gasket will allow oil to escape. A common cause could be a blown head gasket putting too much pressure in the crankcase.
Well you might need to provide a little more information. A car with a slow seep in the valve cover gasket ( Common with older cars ) Will generally run just fine, but it is best to get it replaced to prevent oil burning on the engine. A blown HEAD gasket however is a completely different story. Please add details for extended info.
You have a blown head gasket. Remove the heads and replace the gaskets. Also have the heads inspected for cracks.