Putting oils/grease inside cables can end up leaving the system needing more maintenance. It’ll not only attract/collect more dust, it’ll dry up and thicken over time and require flushing out. You’ll then need to apply more lubricant, which will eventually thicken, slow the system down and require removal. Repeat ad nauseum. In a split system, dirt by itself will not cause much of a problem; giving it oils/grease to mix with will. Cable oilers are the solution to a problem that doesnt exist, or at least it didn’t until the oiler was fitted.
In a split system, dry cables will perform consistently for longer. They *may* have slightly more friction, but they’ll require little or no maintenance. A ‘lubricated’ system will be less consistent and require more maintenance.
A single run of outer cable – or even just at the stops on stays – is by far the best solution, in my experience.