2006 volvo vnl heavy amount of oil going to coolant tank!

I have a 2006 Volvo vnl with a d12 engine with 960000 miles, recently out of nothing, I got a lot of oil going in to the coolant tank. I took truck to the mechanic, and the first thing I was told is the oil cooler, so they took it apart and found out that the oil cooler is fine and instead found oil coming out of the little hole at the bottom of the rear piston sleeve, so they told me the mixing is happening in there and my truck needs an overhaul. any help or advice is appreciated. Anyone had a similar problem? Thanks in advance.

 

I forgot to mention, I had the radiator replaced two weeks ago due to leaking, but is engine oil not transmission oil and is going in heavy amounts like half a gallon in two days... Its a13 speed eaton.

 

So I need an engine overhaul?

 

no, also what is the reason that happen, is it because cold starts? because it happened in the winter.