1qt is usually enough to fill the primary just to the bottom of the clutch spring, which is all you want, and all I've been doing for almost 15 years.
The other day I had my primary cover off, so that ALL the oil was out, then it needed just a few extra ounces.
I reality it's a chain, sprockets and a couple bearings, all of which is just happy as a pig in mud to have any lubrication at all , let alone a full quart of "primary oil". I've run just about everything you can imagine in there, ATF, motor oil, gear lube, 10w-40, 20w-50, synthetic, Dino, etc with no real noticeable difference in any of them. My go-to oil is usually some brand of 80-90w gear oil, seems to work fine other than it stinks a bit, and it REALLY stinks if you burn up a stator.
2011 Merlot RGU, 103", SE204 cams, V&H Dresser Duals, Ness BS, Power Vision, Progressive 940s/Monotubes, 13" MadStad.