You got a lot of good, useful answers already but I did want to mention that once that over tightened, pesky cap is off, it does not have to be tightened again with any real force. Something like a coin held in your fingers is ample to tighten the cap. All the trouble starts when people use tools that have leverage in the first place. The final drive cap as well as the engine oil cap is interchangeable and the engine oil cap is plastic and tightened with nothing other than fingers and they do not fall off or loosen. This is a classic case of using tools where they really are not even needed (not blaming anyone here, just pointing that out). The caps on both lube fill points contain an O-ring that merely has to be snug, not really tight. Once the O-ring is compressed, and finger tight is more than enough, any additional tightening force is simply used to 'mash' the body of the cap into the body of the housing and wasted effort, although it does make it interesting to remove the cap later if 'extra' torque is used to tighten it.
Just snug them up without any kind of tool and all will be well at the next lube change.
Brian
I got my bike used and the plug is already buggered up some. What do you guys use to open it? Looks like you could use a silver dollar and a crescent wrench.
I doubt many people have the driver filler cap tool: 57001-1454.