I rarely clean the old oil from my slow-cooker, is it OK?
You have a couple of potential issues, one possibly dangerous, the other has to do with flavor/quality control.  First, by not washing your cooking vessel, you leave the potential for bacterial growth.  This would be especially true if there is some down time (even a couple of hours) between uses.  The quality issue is that over time residual oil is going to become rancid, introducing off flavors into your food products.  So, I would strongly suggest a wash in warm, soapy water, or the dishwasher, between uses.