I flush my oil tank every spring using paraffin and a big handful of screws and Fasteners
(counted in......counted out)
and paraffin followed by several hot water and soap powder rinses
You'll be amazed how much rust accumulates in a steel oil tank.
If the tank is really bad, you might want to try some other things
I used electrolysis to clean the inside of my petrol tank and I'm amazed how well it worked. All you need is a battery
charger, some baking soda and any improvised electrode for the rust to collect onto. It's worth trying just for the fun