Oily fouled plugs only on one side of engine

I changed the plugs on my 99 Windstar and noticed that the plugs on the front side of the engine were very oily and fouled, while the plugs on the back side of the engine were very clean. Any idea what would cause this?

I am not familiar with how the various lines connect to the upper intake manifold on the '99.
It is normal for some oil to be drawn into the intake...and on into the cylinders through the PCV system.

There IS a TSB (sticky post at the very top of this forum page) for a improved front valve cover that has a improved baffel for the PCV line to reduce the excessive oil that was being drawn into the engine.
So that would be 1 place to check.

In Theory, the oil in the PCV line would be distributed evenly throughout all the cylinders, but if something has happened with the airflow pattern...as in some change as how the air that flows over the PCV line input to the upper intake manifold......then it could cause a change in how the vapor is distributed.
From what other's have posted, there is some sort of inner part to the upper intake manifold......an internal part that can come loose.....and is mentioned as causing a lot of rattling noise.
Perhaps that has come loose?.....would not necessarily cause a noise.....

Also, if you have a slight leakage due to the "isolator" bolts, it could change the airflow.

In short, check into the TSB and see if that is a help.