All good tanks have a drain on the bottom. The question isn’t so much ‘How can I drain oil….", but "what do I do WITH the oil." You cannot drain this oil onto the ground…it will pollute the water supply much more quickly than you think. IF there is a lot of oil in the tank, this is far better handled by professionals with trucks, hoses, pumps, etc. that can take the oil. Please do not take on something you are not prepared to handle. IF there is little oil in the tank, you can pump it into 5 gallon containers and dispose of. Oil does NOT store well, for it breaks down, darkens, and becomes basically useless over time. It also grows an insidious alga in it that will plug burners or fuel jets. IF you are replacing the tank due to age/rust, use a container, drain out the bottom of the tank, for it will have water in it. Transfer the rest of the fuel to the new tank….in other words, I hope you have a spot to set up the new tank. Oil tanks will suffer far less contamination/condensation if inside rather than outside. Good luck
Don’t siphon flammables with a pump not made for that. the metal parts could cause static electricity and ignite the fuel. When you have fuel contained there is a potential bomb. The oil tanks I have seen have a large drain plug that indents in the middle with a nut in a circle on the out side. Chances are this is rusted in place. You may want to drench it in Liquid Wrench or PB (Brands of loosening liquid) for an hour before trying to turn because there is a chance of stripping the nut. Do not drill into a fuel tank or use a torch or saw to cut it open or apart after it is empty. I repeat When you have fuel contained there is a potential bomb. It is the fumes that ignite not the fuel. Try putting an add out on Craigslist. this oil may be able to be recycled for bio diesel fuel. I am not sure about this but it is worth a shot. You can call the fire dept. and they might be able to tell you how to discard old fuel. Some gas stations recycle old oil. There are plenty of resources for discarding fuel. Here in the west we are very eco conscious and would not want it showing up in our drinking water, so please discard it accordingly. There is $5000.00 fine here for improper disposal.
If you have a lot of oil left in the tank you will want to have someone come in and pump it out. If you only have 5, 10 or maybe even 20 gallons you can simply unscrew the filter and use the pinwheel valve to start and stop the flow. You will need to jack the opposite end of the tank up so you can get more oil out. Drain it into safe buckets that can be sealed and then recycle the oil if you are getting rid of your tank and switching to gas or something. If the oil is still good (not black and gooky) then you can pour it back into the new tank.