1988 Jeep Wagoneer Oil blow by into air filter

I have an 88 Jeep Wagoneer with a I6 engin. It runs well but I have a bad problem of oil blowing out of the valve cover and into the air filter. I am loosing a 1/2 quart a week and having to constantly replace the air filter. What is causing this and how do I fix it. I tried to put a breather element in the system and bypass the line to the air filter but the pressure in the valve cover was high and this did not work well.

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