1998 Ford Windstar air filter

There are three possibilities (among other variables, too lengthy to be included in this text):

1:) Clogged or faulty PCV Valve or PCV system. The blow-by gasses in the crank case are not being sucked into the PCV system, thus the only vent available is the blow-by pipe which runs from the valve cover to the air cleaner.

2:) The driver of the car is running the engine real hard. Hard accelleration, high rpm's, etc. This creates positive pressure inside the crank case (blow-by gasses past the piston rings). The only vent available is (again) the blow-by tube running from the valve cover to the air cleaner. The PCV is somewhat negated from the system during hard accelleration because intake vacuum downstream from the throttle plate is little. Thus, the PCV system is not sucking much into the intake manifold during hard accelleration. The only vent is the blow-by tube heading to the air cleaner.

3:) The engine piston rings may be worn, resulting in excessive blow-by.

Simple test: Run the engine at idle with the oil filler cap off. Hang a rag over the oil filler hole (the hole in the valve cover). Observe the rag. Then race the engine (IN TRANSMISSION PARK for SAFETY) up to 3,000 rpm or so. Have an assistant "gun" the engine by sharply opening the throttle. Don't over-rev the engine. Observe the hanging rag. If it's movement is primarily being blown away from the oil filler hole, you know you have excessive blow-by gasses.

Try changing to different oil type, oil weight (for example, if you normally use 10-30, use 10-40 for a trial) and see if any improvement. If you normally use department store oil, try Castrol instead. My opinion, don't bother with synthetic oils; other mechanics may differ.

Try changing the PCV valve.

That's it for my thoughts.

Sorry about the smiley faces after 1 2 and 3. All I typed was a Number, then the Colon : and then the Parentheses. I see the system observed the colon and the parentheses as an emoticon.

Disregard the emoticons.

Check your PCV system, and make sure the throttle body is clean, especially downstream from the throttle plate. There's two or three little holes drilled into the throttle body just downstream from the plate to allow metered vacuum to varioius sensors.
One of these holes is the PCV suction hole: Make sure the airway all the way from the PCV to the throttle body is unobstructed.
PCV valves aren't that much $$. Try replacing it.

Make sure there isn't a lot of sludge or carbon deposits on within the valve-train area. Remember that the PCV valve is on one side of the engine "V" and the blow-by tube is on the other side. If there is an obstruction of air flow within the crank-case, the PCV can pull air all it wants, but there will still be a positive pressure on the other side (blow-by), and this positive pressure will blow thru the tube towards the air cleaner, bringing oil vapor with it.

Also try the different oil strategy I recommended.

Finally, this is a wild guess, but a logical thought:
Make sure the main airway upstream from the air filter is not obstructed. There should be a large snorkel leading to the body panels and/or frame. Around the battery someplace. Make sure there's nothing stopping airflow thru the main air path.
If there is any obstruction, the negative pressure within the air cleaner chamber will pull in air from any available source. The next available source besides the main snorkel is the blow-by pipe, which brings all that oil vapor, which eventually soaks into the filter as you describe.

If everything fails and you can be creative with pvc plumbing pipe and fittings/adapters, create a pipe pathway with a small chamber filled with steel wool which leads to the air cleaner. This way, any oil vapor travelling up the tube will be caught by the steel wool, liquify, and drain back into valve cover thru the same tube the vapor was sucked up. The remaining air (without the oil vapor) will travel to the air cleaner chamber to be consumed during regular engine operation.
Believe me, I've tried this with success. Just make sure you use long strands of steel wool entangled with one another so no short stray strands fall into the valve cover, and get entangled in the valvetrain.