I also have an oil boiler, and a tank in the garden. The tank was fitting with an Apollo Ultrasonic oil level sensor, which sends a signal to base station with a very basic LCD display in the house via 433Mhz radio.

I use an RTL-SDR USB radio dongle, a cheap 433MHz antenna and the rtl_433 software to monitor the signals from the ultrasonic sensor, which transits roughly once an hour. The level measurement transmitted is a fairly accurate centimetre value (I compared it with manual measurements with a dip stick for a few months).

The base station only showed a vague level indication with 10 bars, but now I have more a more precise smart display of the tank level, without any extra modification to the tank system.