Graphite into Oil Paint

Like so many things that come up on this forum, it depends on a number of factors and just what you mean by durable. Graphite in itself is very stable so that is not an issue. Probably the biggest factor here is just how much oil paint are you drawing into. If the paint is substantial enough that the drying oil saturates the graphite particles, the graphite would be incorporated into the paint and would be quite stable.
If the particles are not incorporated into the paint, the particles would rely more on mechanical adhesion. This is itself is not a deal breaker. There are many, many paintings where contours have been reinforced with pencil. The only real problem is possible smearing on one hand or removal during future cleanings on the other. You mention that the work will be varnished, this means that the first potential issue would not be a problem. However, the graphite could possible be displaced when and if the varnish is removed at a later time. This is in no way a certainty. Pencil was sometimes used to depict the rigging on maritime paintings. I have observed some examples where the graphite lines have resisted restoration and others where the rigging is partially removed. To be fair, though, I have also seen plenty of instances where rigging executed in oil paint  (or oil with an addition of varnish) has been removed by the insensitive cleaning of an ill-trained restorer.  
I hate to keep giving less that definite answers, but “it depends” is often the only true answer.

Once a graphite sketch on the ground is integrated into a dry paint film it should be stable, unless the graphite application is so heavy some unbound, loose particles remain. If graphite lines are in the top layers, , the varnish can mix with the graphite particles, and consequently these marks can be lost if the varnish is removed.
Graphite particles don’t migrate through a dry oil paint film, but the material is denser than charcoal and can mix readily with paint, staining light colors in the first application. Also, as paint ages and becomes increasingly transparent, marks underneath may become more visible.
We know graphite doesn’t migrate within a paint film because Albert Bierstadt used a graphite-based oil ground for many of his paintings and, while the pictures may have darkened slightly, the ground color has not asserted itself through the actual paint. Unfortunately graphite primer proved not to provide a very good surface for oil paint adhesion, and Bierstadt eventually switched back to lead white grounds.​

Sorry, Brian, I must have been writing while you clicked “post”. ​

Matthew, no problem, more responses are always better that fewer. We tend to focus on different aspects of the concept and the combination of replies is always richer than would be the case if only one person responded.

To the OP
Drawing into thicker paint should insure integration and be stable. Additionally, even if you do not follow this precaution, your work may still remain relatively stable. Drawing into more substantial layers of oil only ensure this.

​Yes this is true (that graphite does NOT migrate through egg tempera films). Again I suspect that this notion has been propagated by the occasional scenario of the paint layers becoming more transparent over time thereby making it SEEM as if the underdrawing doen in graphite is “migrating” up through the paint layers (when in fact it is simply becoming slightly more visible). While this tends to happen more readily in oil paint films it certainly can happen to a certain extent with egg tempera depending on many things (the particular pigment(s) present, the thickness of the paint layer, etc.).