It is impossible to calculate such risks for any one person; it depends too much on things that are unknown to me, like partner selection (which you already know about) and so on; and which STDs (HIV and chlamydia very low risk; herpes and gonorrhea higher; symptomatic warts low, but asymptomatic HPV probably high; and so on). My guess is the risk of HIV from unprotected receptive oral is in the same ballpark as with condom-protected vaginal sex, i.e. very low, perhaps in the air crash/lightning range.
HHH, MD
What do you consider "very low risk?" Does that mean that the odds are along the lines of being killed in an airline crash? being struck by lightning??
Would you consider this a lower risk activity than protected sex? (taking into account the likelihood of condom breakage/slippage)
Thank you for your comment re: choosing partners wisely. That does help.
HIV infection can be acquired by performing fellatio. It's not common, a lot less risky than vaginal or anal sex, but not zero. (The opposite direction is very safe; HIV is almost never acquired by receiving fellatio.) However, no research has been done to answer your questions in the detail you are seeking. It makes sense that the risk is higher if there are open sores in the mouth or gum inflammation, and ejaculation in the mouth or swallowing semen might be riskier than spitting it out right away, but no data are available.
The main thing you can do to protect yourself is to know something about your partners. If you avoid performing fellatio on gay/bi men or guys who inject drugs, you can consider yourself at very low risk. In that case, I do not suggest HIV testing after any particular exposure; just get tested about once a year. (You also should be tested for other STDs, especially oral gonorrhea.) Of course you also could use condoms, but most people aren't keen on condoms for oral sex.
I hope that helps. Good luck-- HHH, MD