70hp outboard boat motor hasn't run in five years?

If a 2 stroke no oil in the engine if oil injection empty the oil tank and clean filter. Remove the spark plugs and spray a lube like W10 40 and then turn the fly wheel over to make sure it isn't locked up. Remove the carb cover and remove the 3- 7/16 drain plugs on the base of the carbs spray Gumout carb cleaner in the holes make sure the red tube fits in the main jets. Then install the 7/16 plugs and inside the carb there is a tube in the center bend the red spray tube to fit inside the brass tube and spray till it flows out. Let sit overnight. Remove the gas lines and clean or replace. Fuel filter replace and primer bulb. Clean the gas tanks add Gumout . Fill with new gas and oil. Drain the carbs and prime install water hose and try starting. If the carbs didn't clean up they will need to be removed and cleaned.

By all means, replace the gasoline. Oil in the crank can be drained and replaced but it does not wear out unless exposed to constant heat. One of the primary things to look at if it has not been in the water for extended periods, is the impeller in the water pump. They tend to 'dry rot' over periods of time and is one of the first things to eave you stranded. Also check your spark plugs around the base where they screw into the engine (particularly if exposed to salt water). They should be replace annually simply because the metal bands around the base will corrode and allow the ceramic plug to 'blow out' from the engine compression. 'If' this should ever happen to you, "DO NOT" attempt to remove the remaining base that is left in the engine block, without the ceramic stem that was blown out. Find it and tap it lightly back into the base. The base is a thin band of metal and will collapse if you put pressure on the wrench to remove it. It then becomes a major problem to remove it and normally requires taking the engine head off to facilitate removal. Other than these simple precautions, I would check wiring for deterioration, especially around terminal points. Of course, you should always check your battery for dead cells, etc. Once you have checked everything, a good spraying with WD-40 or similar will displace any moisture and help lubricate controls, etc. I sincerely hope this may help you and that you can enjoy your new vessel. Be safe and have fun! God Bless!