My tank came with a mounting point on the top of it where you can fit an ultrasonic sensor. You just configure it with the depth of your tank and that’s it. I have a unit inside that shows how much oil I have left.

Apparently, it uses an RF protocol that can be intercepted and interpreted in HA with the right dongle but I haven’t done that.

Edit: mine looks like this although with different branding.

The only option I could think of would be integrating an industrial oil tank sensor.

A wire sensor that uses Time-domain-reflectometry would likely be the best, but expensive. This uses a corrosion resistant cable and uses wave reflections when a pulse changes mediums (air to oil) in order to give a level reading.

More difficult to DIY though. You have to know what you are doing.

Ultrasonic sensor might work, but it depends on if oil for home heating gives off fumes that would interfere with it.

Otherwise another DIY solution would be optical sensing like a ToF sensor. Maybe the most realistic for easy integration in ESPHome, but like the ultrasonic sensor, you would have to protect it from a full tank contaminating the sensor with oil.

If the tank is plastic, a capacitive sensor could work too.