try removing lens and shining flash light under primary mirror if on and off causes big changes in meter it most likely is mirror.

kerosene is excellent solvent but removing it mission impossible sorry Zippo will flash off but still may stain rear of mirror dont worry we are not going to clean any mirror surfaces... big sin

invest in a can or borrow. spill 1cm of Zippo into cassette tub. Find largest darning needle remove lens set shutter to B invert camera and fire shutter and lock with a cable release to provide access to secondary mirror pivots

Dip sharp end of needle in Zippo transfer to each mirror hinge until bored we need small droplets

then fire shutter on 125 lots of times probable lube turned to gum hopefully Zippo long term fix

repeat darning needle treatment if no effect Id not be tempted to use needle as a pry bar just keep trying