Black oil spots in dishwasher

Every so often I find small black oil spots in my dishwasher and on the dishes. Is this something I can take care of myself or do I require a service call?

Your Model Number does not come up anywhere, please check it.

As far as I know there is nothing in a dishwasher which could cause oil spots.

Only thing I can think of is the unit is siphoning it back out of your drain.
Check that the drain hose is looped up at least a foot higher than where it enters the drain pipe.