Have not done change personally, but have done a fair amount of research and (assuming Honda did not change plug specs) that should be the correct fitment. Are you using the driver or an extension inserted into plug?
Take a sharp tool like a dental pick or sharp screwdriver and scrape all the road grime out of there. I'm doing mine this Fall and I'll bring a wooden mallet to give a breaker bar a little help (gently, of course). As has been said previously, make sure you can remove the fill plug BEFORE you drain the fluid !! Good luck. Craig
Usually a 3/8 will work. However, a few years back I ran into a situation where 3/8 was too big and 1/4 was too small. I ended up buying a tool of the tool truck. The tool guy knew exactly what I needed. Not sure what your issue is, if 3/8 is too big or too small. You may just have junk built up in where the extension would go.