In one or more examples, a method comprises advancing a tubing string along a primary wellbore toward a junction having a low-side exit to a secondary wellbore, with a guide sub positioned at a leading end of the tubing string. The guide sub has a buoyancy within a well fluid external to the guide sub. The buoyancy of the guide sub is used to bias the guide sub toward a high-side of the primary wellbore while moving the guide sub across the low-side exit to a downstream portion of the primary wellbore. Subsequently, the guide sub is used to guide a fluid or tubular component between the tubing string and the downstream portion of the primary wellbore across the low-side exit.