a leaky nozzle can dilute the oil, but is usually accompanied with smoke, and possibly running a little rough. You can also have O rings leaking which is where it was mentioned before about removing the valve cover and running the engine checking for fuel coming up around the injector base..... stay CLEAR of the injector wiring while it's running. If you are in the Phoenix area i beleive Southwest Diesel has the equipment to test common rail injectors. If you pull the injectors yourself, be sure to replace the cross tube o rings too.