I have not been into many of them myself. The early ones had cam roller problems but I do not recall anything like that. Turned every page in the Mack service manual and see nothing like that.
In the last 5 years it had fuel in the oil. The factory locktite coating on a plug appeared to have desolved. That was in the front of a fuel rail.
The turbo was replaced last year but we did not do that.
I am leaning toward it can from the turbo. The oil probably has not been changed since then as it does not run 10,000 miles a year.
Replaced the supply pump leaking fuel into the crankcase. Checked rod and main bearings. Replaced them, polished the crank and test drove it.
It will move some equipment tomorrow.
Found similar bushing in E-Tech Mack once. Couldn't find anything wrong or missing. Older Mack tech looked @ it for a minute and asked "previous turbo failure"? Checked back and sure enough it broke a turbo shaft the year before. Often more worried about air handling and forget about what goes down the drain tube!