Most heat pumps on the market today will beat oil at temps much much lower than 40 degrees. That 40 degree cutover number is from heat pump technology over 5 years old. The industry has really improved low temp efficiency a ton in the last 5 years.

My Bosch unit (installed in 2022) runs with a COP of 2.4 at 5 degrees. With that COP it's still cheaper than oil to run. As temps go above 5 degrees into the 10s and 20s and 30s, it becomes significantly cheaper than oil (as much as 1/2 the cost at higher temps).

Most current Mitsubishi mini-split models, even the non-hyper heat ones, maintain a COP around 2.2 down to 0 degrees or so. At temps above 10-15 F, they're often closer to a 3.0 COP and some make it close to a 4.0 COP in the upper 20s and 30s.