A fender extender will keep a reasonable amount of muck from flying around. It won't do much to stop debris hitting the oil cooler. Whether the debris will ever do much more than bend the radiator's fins is debatable (straighten them with a pair of tweezers *) - or do anything more than make a bit of a mess of flies and road gunk, which will wash off - but a simple guard will work.
The big water radiator is probably safe enough but guards are cheap enough.
* and / or use a small screwdriver, the same diameter as the gap between the fins. When using a screwdriver, always upwards towards the loop.