Duncan has said it about the silvering on the mirror. VERY VERY delicate. The most I have ever done is to sweep with a very fine brush. I would NOT advise using any form of liquid cleaning.
I think you are more likely to find dust that is visible in the viewfinder to be on the focussing screen itself. If the spots you see are sharp then they are on the screen and not the mirror. Just dirtied my F2 to prove accuracy of that statement. ( lunatic
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The fresnal part is very finely etched onto the underside and can attract dust. The top of the screen is a polished surface.
On the older F2 and F3 cameras with a removeable/interchangeable viewfinder dust on the screen was a regular hazard. No worries about changing lenses in dodgy conditions in those days, as the screens were removable and could be cleaned with a blower.
So first thing is to determine if dust is on the screen. Fairly sharp or sharp when looking through viewfinder. If looking at the mirror and you cannot grow spuds on it then most likely on the screen.
Don