If you’re seeing either oil in the coolant or coolant in the oil, you’re going to need a good engine block sealer product. Or maybe you’re leaking compression between cylinders. Depending on what exactly you need to do, there’s a few that come to mind. If you have the first problem described, K&W makes Permanent Metallic Block, and Radiator Seal and Gunk makes Heavy Duty Engine Block Seal. You can find this at most stores that have automotive chemicals. The second problem described would actually require more of a filler than a sealer. Moreso, Engine Block Filler allows you to seal cracks between cylinders, and then bore then cylinders the same size.