I found that using either petrol or acetone will dissolve/remove anything on the coin and when left to dry will leave a whitish waxy looking area in places.this can be seen more clearly the more residue that was on in the first place.Using distilled water straight after putting anything on should help remove it more thoroughly ,put the coin in the water in a plastic tub,swish it around for a minute then pat dry with a lint free cloth.
Better not to do anything in the first place but if the coin is already ruined it cant really appear much worse regardless of what you do đ
Everyone uses different methods including three in one oil for silver which if only a tiny amount may help.
I would think most of the years you mention you can pick up cheaply ,so maybe worth just replacing them before spending a few quid on cleaning stuff .