If an injector or injectors are allowiing too much fuel to enter the intake manifold [this is a leaking fuel injector] (especially when the engine is off) the excess fuel will enter the cylinder, leak past the rings and into the engine oil.
Its kinda hard to check for leaking injector, but start by having the computer codes read. It may have a misfire code stored. If so, this will give you an idea of which cylinder (injector) you should check. Let me know how it turns out and I can give more info on checking the injector.