I suspect the Systane product line has more published studies than any other over the counter tear.

Novel Ocular Lubricant Containing an Intelligent Delivery System: Details of Its Mechanism of Action.
Dev Ophthalmol. 2010;45:139-147. Epub 2010 May 18.
Springs C.
Department of Ophthalmology, Indiana University, Indianapolis, Ind., USA.

Objective: The purpose of this review is to outline the mechanism of action of a novel ocular lubricant incorporating hydroxypropyl-guar (HPG) and the demulcents polyethylene glycol 400 and propylene glycol.

Methods: The literature relating to the mechanism of action of Systane Ultra is presented. The literature search covered the period prior to June 2008. A manual search was also conducted based on citations in the published literature. Additional original reports were referenced if relevant to the subject matter of the review.

Results and Conclusion: The published literature supports the efficacy of an ocular lubricant containing HPG at reducing the signs and symptoms of dry eye through its multiphasic behavior and duration of action. This new formulation presents an additional benefit of a delivery system to further reduce the assimilation time of the product on-eye. Beneficial outcomes have been documented for tear film interaction, blur profile, viscoelasticity and tensile strength. Copyright © 2010 S. Karger AG, Basel.