My understanding (and I'm not a doctor, but my own own doctor is an HIV specialist, and I work with HIV agencies that disseminate this information) is that the risk from oral is low, but does exist. The US CDC confirm this. Yet the clinical evidence is weak, mostly anecdotal reports.
The danger appears not to be from swallowing, but in the mouth. Where open sores and gums may offer entry points for HIV. I've always been told not to brush my teeth for several hours prior to oral, just use mouthwash to freshen my breath. And I do have bad gums, often bleeding a little after brushing.
I've known some guys who only perform oral with the other guy wearing a condom. I did that myself with my late partner because I knew he had HIV. I'd wipe down the condom with a damp washcloth to remove any lube and spermicide.
Such precautions aren't necessary with my husband & me today, because we are 100% monogamous and tested several times a year for all STIs, during routine blood work. So I can "bareback" him in my mouth.
Hell, he can bareback me up the ass, but that's only because we've been monogamous together for over 7 years, and take blood tests all the time due to other medical issues we have. If you don't know the guy that well, however, and wanna trust him with your life, Safe Sex means assuming he's got something, that even he may not know he has, and taking precautions.
So again, oral sex is believed to be low risk for HIV, but not without any risk. How you manage that risk is up to you.