seized slant six

Or try kroil, or pb blaster, some kind of penetrating oil. Fill the cyl to cover the top of pistons to cover the rings all the way round. I'd let it sit for a few days and keep sticking oil in it. Then I would remove the valve cover and squirt some down the stem of each valve, pre oil if you want. Break it loose with a bar then with the plugs out crank it over for a few seconds, spray oil down again and crank it a few more seconds this will blow a lot of crud out. Good luck