I think we've been here before just over 2 years ago and your Leder Gris was 5 years old then?
http://www.walkingforum.co.uk/index.php?topic=31385.0As mentioned in that thread the petroleum "oil" content has probably just naturally evapourated/dried out over time and needs to be replaced. The Leder Gris tins aren't airtight like the screw top jars now used by some of the other wax manufacturer's which is causing the problem for long term storage.
It may be worth chucking it in an old pan and slowly melting it then pour it back into the tin and use but as the oil content is the active ingredient which goes down into the leather and replenishes it you may find that the wax just sits on the surface of the leather and offers no real benefit.
You may be on a hiding to nothing but if you've got any of the softer waxes like Meindl Sportwax or Berghaus/Brasher leather conditioner it may be worth mixing some of it in with the Leder Gris to try and replace the oil content. Adding a bit of Vaseline Petroleum Jelly may be worth a try if you've none of the other softer waxes kicking about.
The cheapest place I can find for Altberg's Leder Gris Original Clear/Natural 80g Tin is here
http://www.odintactical.co.uk/leder-gris-boot-wax @ £4.50/80g tin with free delivery.
It's probably about time that you treated yourself and your boots and let the moths out of your purse! Not Scottish are you?