It's an interesting one. If you lube your cables regularly, you can use anything really, WD-40, engine oil, whatever. Because when the inner sheath breaks down, it doesn't matter so much because the cable is covered in oil. But you'll have to keep doing it. If the cable is already stiff or notchy then the inner sheath is gone and you are metal to metal in the cable. Lubing it will help short term but you're really better off just buying a new, genuine cable. They're not that expensive given how long they last and it will give a much smoother action than a lubricated worn out one. I have lots of experience with this over the years but now, the only lubricant I would put in a cable would be silicone-based grease or spray as this is the accepted lubricant for nylon/rubber wear points.