Funny you should mention this, as I actually do a part time jobbie at a Garden Machinery Center and we have untold grief with stale fuel
Since we have gone over to unleaded the fuel shelf life has dropped considerabley, as well as the chemicals they now bung in it affecting some of the plastics etc.
But theres also another problem is that the plastics they are now using are very cheap and horrible and get attacked by fuel easily.
Another problem we've found especially with the cheap chinese stuff is they REALLY dont like old fuel, the reason we feel is they run them so lean these days to meet emmision regs (yes lawn mowers etc have to meet emmisions these days) that once the fuel has "gone off" theres just not enough flamable energy left in the tiny amount sucked into the engine for it to actually fire up on.
Take stale fuel and put it in an old engine and it will run fine, try it back in a modern lean burn and it wont
To be fair I suspect the heaters should be fine on unleaded as they would have been made for it, but the fuel does go stale quickly and hence why its not good to leave a lot in the tank during the summer period.
ChrisB