The Perkins 6.354 is very straight-forward w.r.t. the fuel path. It goes into the lifter pump, out of the lifter pump, through the filter, into the injector pump, and from there into each injector, and from there out of each injector into a common return rail, and from there back to the tank. There is also a bleed path from a bleeder hole in a banjo bolt at the fuelcfilter back to the tank via the return rail. All outside of the engine block. No internal passages. None.
Fuel can get into the engine only where this path interacts with it: each injector, lifter pump seal (interaction because the engine drives the lifter pump, not fuel path), injector pump seal (interaction because the engine drives the injector pump, not fuel path).
Fuel can make its way into the oil via exactly one of these interfaces or via the normal combustion path, in normal or abnormal volumes.
Given the volume of fuel, without heavy white diesel smoke, it is dramatically unlikely that injectors are dumping enough fuel into the engine to cause this.
Without a huge amount of blow by, and likely also a smoking problem, it is pretty unlikely that the engine has an internal problem, e.g. with a ring, bad enough to cause this. Having said that, I've see. These engines seeingly run surprisingly well on 5 cylinders.
If there is a leak or crack or hole along the supply or return path to or from the injectors, is into the bilge. These paths are all external to the engine. A return leak can not cause this problem. The path isn't near oil. I guess the exception would be if the valve cover were off or if the oil dipstick were out and it drained from the engine or area around into the engine. Or, maybe, got sucked into the air intake. Or something like that.
But the engine room would bebred and slimy beyond doubt first.
An /obstructed/ return path, allowing fuel to back up and pressure to form in what should be a free flowing path, could conceivably cause an injector problem or temporary or permanent seal failure that could possibly cause this. My model of this failure mode isn't super good. Something would have to give or misbehave. If it was at an injector or injector pump seal, it could be a problem.
If the return checks good, I'm betting the shop finds and fixes the problem at the injector pump. While you are paying them to rebuild the pump, if you want, you can ask them to pop test the I jesters for free. Shops do that as a courtesy.
If that doesn't do it, it is leak down test time. But it is hard to imagine that type of problem without a lot of blow by.