First, I'll give you some links for 'stuff' you can use to 'stuff' those gaps. (I don't know what you mix w/sawdust to get it 'to do what you want it to do' ...) Visit Google.com WEB Search Engine and peruse the links. You'll see from the 'hits' that gaps are 'talked about' and solutions addressed in the links. If you use your Browser's Find/Search feature (which can be found on the pull-down menu), search for gap on each of the link's WEB pages.
Now, you may be aware of what I'm about to tell you but for the benefit of others, I'll say it anyways. Personally, I don't recommend any filler for gaps. Gaps are the result of 'movements' of the wood when it comes to expansion and contraction. If the floor is allowed to continue expanding and contracting, your 'filler' will eventully work itself loose, and out, creating gaps again. And if it gets 'compressed', it won't flex back to its 'natural state'.
By controlling the humidity levels in the home, you can control the movement of the wood. It's that simple. High humidity (or high moisture levels) cause the wood to expand. Low humidity keeps the house dry and allows the wood to contract. So if you live in an environment where you have changes in humidity from season to season, you'll have gaps UNLESS you can control it in the house.
If the room under the hardwood floor is 'different' in terms of humidity and temp levels than the room that has the floor, you'll still have the problem. The KEY is to try your BEST at getting the room below as wall as the room w/the floor to be the same. Some floors can tolerate a difference in the levels but in those cases, they aren't like 'night and day'. (I mean, I know of homes that have warm, damp basements but cool, dry living quarters. This may be OK if levels drop naturally but if they don't, the gaps return.)
If you need more info or guidance, post back. My best to ya and hope this helps.
Jay J -Moderator
PS: You can try the filler. It might work. It might work for a while. I'd stay away from any 'home remedy' if you want your results to last. (But then again, EVEN the best stuff off-the-shelf may not work or last ...)