I've only had my L4060 about a month, so certainly not long enough to judge yet. With that said, fuel consumption has been wonderful. I'm almost ready for the 50 hour service and most of that time has been bush hogging at 2500 rpm with the rest dirt work back and forth, hydrostat response +2 and auto throttle advance. I've used almost two full tanks (26.4 gallons per the odo). Have not done a regen yet though. I suspect that'll burn up a bit of fuel.
As for the filler neck it's more than protected by the FEL. I never remove mine so I have no worries about knocking the filler off. Plus, the top of the neck is connected by a flex hose coupling and clamp above the tank so I would expect anything that hit it hard enough to damage the filler would come closer to knocking the neck off, leaving the tank unhurt.