2250 Oil in Coolant.

[5cylinder]

Hello, I have a 2013 Peterbilt with the 2250 engine that is pushing oil in the coolant. The oil cooler was removed and tested before I got the project. The cooler tested good but was replaced. Couple months and 4k miles and the same problem is back. I pressurized the coolant system to 20 psi and removed the turbo drain line to see if the seals in the turbo let go. (thinking that if the truck was off the pressure would go the other way). System held pressure over night to 11psi and nothing came out of the turbo. Removed the head and sent to a machine shop for testing. They said it was good. Removed the pistons and liners to inspect the liners. O-rings showed signs of breaking down but I think its just from the oil in the cooling system. No cracks and and abnormal wear at all. I've checked with quickserve and not seeing any more troubleshooting steps to go on. Anyone got and more ideas to check. I really don't want to inframe this engine and then have the same problem return.