How to keep inner-cabling clean?

OK fair enough Welsh boy, apologies GT85.

It's a very light oil that will dissolve the thicker oils on cables etc and thereby 'degrease' them.

All compounds that oils will dissolve into - from very light oils to agressive solvents - are essentially on a spectrum. You can degrease your hands after bike repairs with olive oil, for instance.

My mountain bike has exposed cables, just wash it down, don't worry. Cables last year's and are cheap to replace when needed

Does the cable appear again under the bottom bracket? If so, dont worry, the dirt/water will fall through.

If not, is there one of those thin inner liners around the cable here? You can always add a flexible waterproof boot if there is, one of these, as there usually is where the cable enters the loop for the rear mech:

http://www.sjscycles.co.uk/shimano-deore-xt-m739-brake-boot-8aa-4200-eac...

http://www.ebay.co.uk/itm/2-x-Bike-V-Brake-Rubber-Boot-Cable-Noodles-/17...

If not, don't worry too much, a tiny bit of dirt does get inside the frame, but the bottom bracket is sealed so it can't really hurt anything.

I wouldn't use GT85 in a place like this, its basically a degreaser. You don't want the cable to lose its original coating.