Review on Revive And Protect Your Oak Surfaces With ColorSeal Oak (5.5 OZ.) by Nap Olivas

This is the best filler I've found, for gaps in oak floors. It's good because it's dense (not soft and rubbery like caulks), but flexible (not rock-hard like putty and conventional wood-filler, which subsequently cracks). I found the best approach was to over-fill the gap slightly, and press and smooth with an old credit card, leaving a slight excess. Then in about an hour (when the filler is still a little flexible), come back and clean up the excess around the gap by scraping with the credit card. Also scrape over the top of the gap if it's too high, and re-settle any torn fragments with your finger. A credit card is good because it's not hard enough to scratch the surface of the oak, and has a uniform square edges for accurate smoothing. I use it angled up from the horizontal by perhaps 30 degrees, with a compressive motion to smooth the filler, and reversing direction to make a chiseling motion to remove excess.