Take it to a Cummins DIESEL shop. They have the equipment and the
EXPERIENCE.   The DC garage does not work on  Diesels except during a
blue moon on the fifth month of the 6 th year from who knows when.

Mechanical fuel pump or injection pump.  Duh,  only places it CAN get in.

Could have injectors leaking by. In any event 50% dilution is not good. You
might think of getting a oil sample and check to see if there has been any
bearing damage.

Roy

It sounds like they have no clue what they're doing.  Replaced the head?
Last I checked, fuel doesn't flow through the head.  It's POSSIBLE the pump
could be leaking, and letting fuel get into the front cover, and therefore
the oil, but it's more common that one or more injectors is leaking, and
dumping way too much fuel into the cylinder(s).

You say the oil's getting diluted with fuel.  Does the level in the
crankcase rise, or is it the same level but just diluted with fuel?  What's
your fuel economy like?  Do you idle the truck for long periods of time?

If it were me, I'd ask (firmly, but politely) that a new engine be procured
and installed, because of possible damage done to the internals due to the
oil dilution.  Barring that, I'd ask that they allow the truck to be taken
to an authorized Cummins dealer for repair, since they've already had three
shots at it and haven't fixed it (assuming the injector pump swap doesn't
cure things).

There have been a few (very few) reports of leaking injectors over at
http://www.turbodieselregister.com - I've never seen a report as bad as
yours sounds, though.

I had this happen on my 89 some years ago, it was the injection pump
drive shaft seal. The seal cost a few dollars, to replace it cost
hundreds of dollars due to the R&R labor cost. I set my engine on TDC
and the removed the pump myself. Took it in and had the seal replaced,
cost me $60, then reinstalled it myself. No more fuel in oil problem.