There would not be anything "instant" about it. If it is caused by rings then they could have been degrading over time, but are finally to the point where they are letting oil blow by.

Was the oil there prior to the valve cover change? As mentioned earlier, you typically will not have oil coming out of your exhaust if it is leaking on the outside, only an internal leak into the combustion chamber. Oil leaking on the outside of your engine onto the headers/exhaust would cause the smoke to be within the engine bay out not out of the exhaust. Unless the leak is so bad that it is leaking enough to cause a substantial amount of smoke in the engine bay that flows out to the back of the car when you are driving and not really out of the exhaust.

 

i do not know if its comming from the exhaust or not all i see is a white cloud behind my car because it only does it at high rmps under load does not do it it if i rev it or slowly build to high rpms only full throttle