The WIX 51458/NAPA 1458 do have the anti-drainback valve. When you initially changed the oil, then you also removed whatever oil was left in the filters. I am ASSUMING that the spin ons you removed were also anti-drainback type filters. If they were not, well.....
Then you put new oil in, installed new filters, ran the engine, "lost" the oil that it took to fill the filters and then adjusted the oil level, correct? If that tallies up to 20 quarts, so be it....it usually is slightly more, like 22.
The difference between FULL and minimum on the dipstick is something like 6 quarts (not sure offhand, but something like that), so I would not fret about that small of a difference of 2 qt (provided your calculations are all correct!).
Also, there are other variables, such as for how long did you let the oil pan drain? It can go on dripping for a long time, depending on ambient temperature, oil viscosity and whether the truck is really level or not. Oil can sit somewhere in a pocket just because the angle happens to be just right. It is unlikely that you get "all" the oil out. Even a dented oil pan can account for a quart!
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