Fill or no fill
Just reading some of the replies, i guess it depends on the look you want for the floor. is the Oak board you have chosen clean or rustic grade, does the board have bevelled edges?.
If the boards are square edge then trowel the filler on to fill header joints and board edges. This would be the desired way to go on a very clean FAS grade board, If the board is rustic and knotty, as we Brits seem to love then choice is yours. We sometimes fill with a course dust, say 80 grit and wax oil or resin but push the filler in the knot leaving it depressed. This then looks like a "worn" dirt filled knot, great if its a distressed floor.
Normally you would fill with dust from the sanding on a 120 grit. Its easy to cut it back and won't leave a residue.
As for the finish, remember a lacquer is a lot harder to repair if the floor gets damaged, a hardwax oil is so user friendly. Try Floor service by Overmat from Holland. Its a great product or Treatex.
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