I think a Pi is perfect for this kind of thing, but there is some engineering to do.
As mentioned above, tampering with 240 VAC brings serious safety issues, but even 24 VDC and 12 VDC are not without issues. And a Pi can take only 3.3 Volt DC for input, anything higher than that will fry the SOC.
But there are current and voltage sensors that are perfectly safe to use (example: https://www.tinytronics.nl/shop/nl/sens ... 3-000-100a
Your first question was about measuring oil consumption. Measuring the weight of the tank would be the best way to do this, but probably not very practical. Some kind of level sensor is what you need here. Mounting a sensor inside the tank is probably not a good idea (you don't want any electrics inside an oil tank), but there might be sensors that can measure the liquid level from the outside. Maybe a flow sensor on the oil line from the tank to the boiler is a good solution.
So you need to look for sensors that can be used in your particular situation, and you probably need to do some Analog-Digital conversions as well. Everything you need is out there, but you need do some searching and some engineering to make it work.