IMHO, it varys a little depending on how big the filter is.

i would use 5 and top it off to the full mark, I have found over the yrs not all truck pans are 6 qrts some are 5 and use a large qrt filter.if it gets a small filter it is over little over full.

I never trust any of the sticks on used trucks. I change oil + fliter with 5 and either remark or top off..which ever looks right.

6 qrts on my BB 440 rear sump pan puts oil on top of windage tray. so it gets a napa gold 15155 filter and 5 qrts. (I mocked and measured it up before install so I know this)