In that engine the points where diesel can enter the oil are the injectors. Since the injection pump when the seals are seen, the diesel is normally out to the distribution strap or out through the accelerator command axis.
If you have been an open injector (needle attached to up) you will notice it because the engine heads and bells as if it were entering ether by admission since the start of injection is advanced a lot, drips when closing and the spray is very bad.
Anyway, if you have done butts, when lifting it, a piston would be seen a precamara and a cylinder different from the rest and normally they are sent to check the injectors when this type repair is made to the engine to avoid subsequent damage.
When an injector is stuck, in this type of injection systems the first thing that tells you is the presence of black smoke in the exhaust, and a rare noise in the operation.
All the best.