Did you handle your coolant chemistry properly, and/or did the previous owner do so?

In the Navistar/IH engines, you need to check the coolant chemistry every 15K miles, and keep the SCA (supplemental coolant additive) level in the range indicated by the test strips used for your SCA chemistry.

If you don't do this, eventually (usually between 100K and 250K miles, depending on how hard you push the engine), cavitation erosion pinholes the cylinder walls and you start seeing coolant in the engine oil.

The only way to fix this is a new engine block or a new engine. The Navistar engines don't use bore liners/sleeves.