That's a poor milling issue. You can't fill your way out of that.

I’ve read comments on Houzz before re: filling gaps between prefinished hardwood floors (and engineered prefinished floors). I think there are some different ways in order to do this. I am fairly confident that a pro will come along and give you some advice.

Yes, it looks good overall, the thickness will not let a dime fit into the space. We just have some places where there is some sawdust that drops into the groves and leaves lines visible. We are just trying to see if we can do something to help on this

Woodwise and DuraSeal make prefinished fillers for this very purpose. They come in a variety of colors and are the consistency of clay, then harden after application.

Tim, thanks for the information. Greatly apprciated.

Tim, as an additional question. When looking at the mentioned products, it occurred to me, that based on the overall look, I am somewhat indifferent in actually filling the cracks, my biggest concern is that we do not have a spill and then have the flooring expand due to a water issue. Do you have any experience with just using a clear sealer injected into these small cracks?

Spills will work their way in between even the tightest fit planks. It is not a waterproof floor, ever. Your assumptions are incorrect, and your concerns are misplaced. ,

I kind of agree with Tish. Regular maintenance should be done with minimal water so that won't cause any issues. If you have a more substantial spill, water will get into the joints anyway. If you have something really serious, a wet vac could be employed to remove as much moisture as possible, as soon as possible.

Considering the color of your floor, if the open joints are no worse than what you've picture I wouldn't fill them.