Tibetan Medicine is mostly Ayurveda, with some influence from TCM. In fact, I have found it better to study original Ayurveda or TCM rather than Tibetan medicine, and this comes from my good friend Namdrol (who I bicker with all the time, and then reconcile lol), one of the current day experts of not only rDzogchen but also Ayurveda and Tibetan Medicine.
There are two perspectives of mdangs and gdangs - one is where these are concepts borrowed from Ayurveda and its predecessor Samkhya; the other is rDzogchen where these two mean Radiance and Luminescence and have no connection with Ojas.
I had to conserve body fluids only when involved in Karmamudra practices with a consort (which we call the Bhairavi Chakra from the ancient Shaivite practices adopted by Buddhists first in Eastern India and then transported to Tibet after the decline of Buddhism post Islamic invasion) - but once a higher stage is attained, there is no longer dependence of transmutation of energy on loss of bodily fluids as the operation is now on a totally different plane. Hence, a rDzogchen practitioner now calls it the Radiance and Luminescence of the awakened mind rather than bodily energy. Rules are entirely different based on whether one is practicing sexual alchemy, or something else.
In terms of Tantra, at this higher stage, the Ojas is not a function of the individual self anymore, but rather the Ojas of the awakened archetype (deity). Initially, this ojas is mixed - operating between one’s own Ojas and its divine counterpart, but upon stabilization, the ojas of the deity pervades. This is where some higher levels of siddhis (accomplishments, powers) start appearing…