Hey thanks for the photo. That location is right in the area where I had been moving things around. I had completely removed the airbox, and taken the nuts off to remove the intake manifold (but just so I could move it into any position, I didn't completely unhook everything and take it out).
But the vacuum reservoir that is right next to the oil gauge sensor, I had removed the nuts securing that so I could move it to the side and allow me to secure the flexible hose to the coolant crossover pipe.
Since I was moving that around, I think its possible it damaged the wires coming out of the top of the oil gauge sensor.
So I think the F40A error mentioning the oil gauge sensor is because it simply isn't receiving any signal from it, so its as if it thinks the sensor isn't even there, which would explain why it won't let you clear the code.
Obviously now I need to check the sensor connector, but considering the location, I would have to take everything off that I had to when changing that crossover pipe, and that is so much hassle that I really want to avoid that.
I've checked the workshop manual and it mentions to change that sensor, you simply remove the bottom engine protection tray and get it on a hoist, then from that raised position you can get to it from underneath instead, has anyone changed one of those sensors before? Its very cramped back there, how easy is it to access it when hoisted up? Would you have clear line of sight to see if the connector is still on or the wires running to it are damaged?