Unless Bosch recommended it, it doesn't make sense to me. Dishwasher pumps are by design...water lubricated. I really don't think anything will dry out. But the problem you might have is odor from stale water which does remain in the sump. I would suggest pouring a gallon of white vinegar into the bottom of the DW before leaving, running a cycle, then pour a cup of vinegar in before leaving the house. repeat when you return.
It seems to me a cup of cooking oil is must going to gunk up the whole inside of the machine.