I have the 300r, broken in by now with 1500miles. I always calculate my MPG just as a game to try and beat it next time. I do however run it pretty fast. Fast accelerations, I look for nice curvey roads. The best MPG I have gotten was 95mpg. About 15-20 more than what Honda states. I have done a lot of mods also tho. Lightened it up quite a bit and a nice flowing exhaust, air filter, synthetic oil filter and Amsoil synthetic oil. I think that's a pretty darn good MPG for the R.
You could get more by changing your sprockets to the max the CB300 can take (15/34t).
Though I can't speak for a CB300F yet, my Honda Rebel normally takes 10W40 oil, but I found it works better in Hot South Florida, with a few oz of 5W30 in it. It runs smoother, and MPG goes up. So does top speed.
I recently changed that 5W30 to 5W30 Dexos oil, and it's a huge improvement over synthetic or regular motor oil!
It seems 5W30 Dexos oil gives me less clutch slip, yet runs the engine smoother than any other oil!
Right now I do between 2 to 3k miles on my oil, whereas before I did half of the miles on it before I needed an oil change.
The Rebel doesn't have an oil filter.
I'm still in test phase, running 2 stroke oil in my fuel, at a ratio of between 100:1 to 200:1 (160:1 being the sweet spot), and it seems to work great on a 4 stroke!
No plug fouling whatsoever yet, and I've logged over 4k miles on the bike doing that (9k before I started testing with fuel oils)!
I might later on change to cheaper oils, and see how they run.
Better mpg's at lower speed, better top speed; I'm also running the engine quite a lot at 4-6k rpm.
I know my lawnmower (aka test subject) didn't like engine oil in the fuel as much, as it was running less good than with regular fuel (as opposed to it running better when adding 2 stroke fuel); so I'm not even going to try running my bike with engine oil in the fuel; however I did find that the bike also runs better with octane booster; an oil based lubricant, that seems to me overpriced 2 stroke oil without as many chemicals in it.
But also running with engine oil there was very little, to no plug fouling going on.
So far, I can recommend adding 2 stroke oil to a 4 stroker's fuel tank, at 160:1 ratio; if your aim is either improving performance, or improving MPG. Also, supposedly, improving lifetime of the engine.
You're not particularly improving your wallet doing this, as at a 160:1 ratio of oil in the fuel, you're paying about 20% more on fuel, but it should be offset by paying 2oz less fuel per tank, and by having about a 5-10% better MPG, meaning you'll go 5-10% further on a tank.
Financially, you should lose about 5% on the whole, but it does give you the benefit of adding top speed performance and potentially engine life!