To change the filter you will lose about half the fluid in the system. As Scotty mentioned, after putting the new filter in and pan back on, it will take up to 8 quarts to get the level back to normal and you'll have diluted the fluid to 50/50 old/new. You could also add the Lucas as part of the refill.
This is the procedure most people would recommend in your current situation. Also, check that power steering fluid - maybe all of your whines will be gone
This is the procedure most people would recommend in your current situation. Also, check that power steering fluid - maybe all of your whines will be gone