i would start by soaking it in barkeepers friend for a day or two. i take one of those fiber looking scouring pads and cut them in strips then fish them into a straightened coat hanger that i looped on the end and use this to clean the inside of the bottle. you can angle the coat hanger anyway you need. it works great on soda bottles. if the stuff inside is sticky like dried oil or tar you might try letting it set with some mineral spirits over night.
you can also use a toilet brush cut one end loose and makes for a long brush. I am in process of cleaning 400+ bottles that were buried under floor of an old soda pop factory. They are bad. I have gone through 2 toilet brushes already. they are so dirty inside im cheating and hooking it to a drill and spinning it that way. You can get them at the dollar store cheap enough just get the type that is a looped wire not a ball. I agree about bar keepers friends it is great I swear by it. It is one of the best secrets I learned about and shared here.
Brian
soap and water, your local head shop has brushes that are good for bottle cleaning, for stuck on crap id use chemicals, toilet bowl cleaner works well, BUT WHEN USING CHEMICALS YOU MUST PROTECT YOURSELF ! WHERE RUBBER GLOVES AND A MASK ONE TIME I GOT REALLY HIGH OFF TOILET BOWL CLEANER IT WAS NOT GOOD SAFETY FIRST MIKE