Oil Filled Radiator Heaters use the same amount of electricity as a fan heater does and so they are not cheaper to run.
Although the oil filled radiator heater heats up the oil and then the heating element shuts off until the oil cools down some and then it turns back on again to heat the oil up again.
So they may use slightly less electricity than a fan heater that is running constantly but not much.
The oil filled radiator slowly heats the room but a fan heater can heat the room faster.
Both fan heaters and oil filled electric heaters use 1,500 watts of electricity on the highest setting.
Electric fan heaters and oil filled electric radiator heaters can be left on overnight.
Just make sure there's nothing flammable around or near the oil filled radiator heater and make sure there's no curtains etc close to the radiator heater either.
Also make sure the plug fits firmly in the electrical socket and don't use an extension cord with the electric oil filled radiator heater either.
As long as you follow those safety rules then you can safely leave your oil filled electric radiator heater on overnight.
I have an oil filled radiator heater in my bedroom that I leave on at night because my bedroom gets a bit chilly at night especially when the temperature outside is below freezing.
So I leave the electric oil filled radiator heater on all night to keep me and the room warm and it's never been a problem.
It's quiet and heats the room up very good.