I tried Viagra several times just to see whether I would receive harder and I didn't notice any difference in lubrication but then my penis was inside my wife's vagina so I really can't be sure if there was actually any difference. I did get a harder erection though. I imagine you were masturbating whether you noticed more wetness so maybe the Viagra did cause you to have more arousal which is what produces the lubrication or pre-ejaculate, most of which comes from the Cowper's gland or bulbourethral gland.
It may possibly cause your body to produce more lubrication.