My brother gave me his old Pentax K-1000 camera today but the battery has leaked and the cover is stuck shut. I can't unscrew it without risk of stripping the slot.

What do you think I should use to unstick the cover?

Since the battery electrolyte is likely to be alkaline, I could use a weak acid like vinegar. I've also got various types of penetrating oil, carb cleaners, Stoddard solvent and/or WD-40.

What's the best thing to use which has the least risk of causing more trouble than it solves?