Every month or so I keep getting the low oil light, I have to add about a quart and a half to fill it back up. I have brought it to a few different places which all say they don't see a major leak anywhere. Recently I have been getting dark smoke from my exhaust which never used to happen, and now my oil consumption is getting slightly worse. I am going to have my valve cover gaskets replaced this weekend, is there anywhere else I should have the mechanic look that could be the cause? Where do I start, what is usually the cause if not my valve cover gaskets? Thanks