If you even suspect the engine crankcase oil is contaminated with fuel, you had best solve that problem 1st, running with fuel thinned oil will cause an engine to fail. Check the condition of the oil carefully, see if you can detect by it's viscosity and odor if it is diesel fuel.
Don't quote me and I am sure to be corrected, there is a seal or something that fails on the injection pump. Again, do not quote me, but there does not seem to be any other way for it to get in there. Even if I am wrong, you'll want to take care of this before you run it any further. Gaining fluid in the crankcase is never a good thing.