Gas cap vary hard to open

I have a '91 740 with about 135K miles that is in pretty good shape but in the last year the gas cap has been nearly impossible to unscrew. It is the original green Volvo cap without ever being cross-threaded or abused. I am not sure if a strong vacuum seal has caused this or not. I do not hear a hissing sound of air entering or exiting the tank when I finally do get it opened. Should I try to lubricate the threads and seal? Could there be something else going on here?
Thanks,
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