A medical paper published in 2002 documented over 19000 unprotected oral sex encounters where 1 partner was infected with HIV and not a single person contracted the virus.
Authors
del Romero J, Marincovich B, Castilla J, García S, Campo J, Hernando V, Rodríguez C.
Journal
AIDS. 2002 Jun 14;16(9):1296-7.
Source: http://www.ncbi.nlm.nih.gov/m/pubmed/12045500/
The CDC issues safe sex guidelines and takes the stance that there is risk associated with unprotected oral sex but at what risk is unknown. The CDC does not present any real medical research or studies that back up the risk warnings about un protected oral sex.
The university of California at San Francisco states the following as the risk of contracting HIV from oral sex:
There are a few known cases of people getting HIV from giving oral sex (licking or sucking). There are no known cases of someone getting HIV from receiving oral sex (being licked or sucked). Experts believe that oral sex without protection is less risky than other kinds of sex, but all agree that it is possible to get HIV from giving oral sex to an HIV-infected partner without protection, especially if the HIV-infected partner ejaculates in the mouth. Certain factors, such as the presence of any cuts or sores in the mouth, are thought to increase the riskiness of oral sex.Source:http://hivinsite.ucsf.edu/InSite?page=basics-00-08
On March 14, 2003 HIV InSite convened a panel of San Francisco experts to discuss the data on risk of HIV infection associated with receptive oral sex.
Laurence Peiperl, MD (convener)
Director, UCSF Center for HIV Information
Tom Coates, PhD (moderator)
Director, UCSF Center for AIDS Prevention Studies
Participants: Susan P. Buchbinder, MD; Frederick M. Hecht, MD; Jeffrey D. Klausner, MD, MPH; Dennis Osmond, PhD; Kimberly Page Shafer, PhD, MPH; Eric Vittinghoff, PhD
It was concluded that you have a 1 percent chance of infection if you receive oral sex from an HIV positive person.
Source: http://hivinsite.ucsf.edu/InSite?page=pr-rr-05