Airscrew, if it were me I would be tempted to see how much the shop would charge to drop the tank and drain it. It's not usually that hard. I'd run it down to 2-3 gals and then have them drain it empty and re-fill with fresh and a new filter.
As far as running down the tank IMHO 3-4 gals is well safe - the pick-up pipe shouldn't be in the bottom but a bit above.
