Hi XXXXXXXXXXXXXXXX;
The intake maniifold is a possibility. Albeit a slight one. Most likely you have a blown head gasket or a cracked head/block.
The best way would be to disassemble the block and have it hot tanked. The most practicle way is to drain the oil, change the filter and then refill the crank case with fresh oil and a quart of marvel mystery oil. Run the car till it is warm. Drain the crank case and change the filter again. Fill and your good to go.
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Auto Mechanic
11years auto mechanic 30 years grease monkeying around with cars, trucks, boats, etc.