Universe Sandbox can simulate non-spherical gravity as an experimental mode, activated using the Enable Non-Spherical Gravity in the Debug section of the Game Settings Menu.
Normally, N-body simulations like Universe Sandbox assume that each object in the simulation is a perfect sphere, attracting other objects directly towards the center of this sphere with the same force, regardless of the direction. However, in reality, most stars, planets, and moons are not perfectly spherical, but slightly squashed, and have some non-zero Oblateness representing the amount to which they deviate from a perfect sphere.
This non-spherical distribution of mass means that the gravitational force attracting nearby objects can have a different strength and even direction depending on where the attracted object is relative to the shape of the attracting object. For example, an object directly above the pole of a non-spherical attractor would feel a slightly weaker gravitational attraction than an object directly above its equator. And an object above a latitude between the pole and equator would feel a gravitational pull not towards the center of the non-spherical attractor, but also slightly towards its equator.
This variation in the force of gravity can alter the orbit of the attracted body, causing its orbit to shift or precess over time.