I can't believe I clicked the link. At work.
Anyways, all good advice is upthread. For me I found that I sweat profusely and it's the dried salt crystals that can cause chafing between the cheeks after >100 miles.
My solution is nothing more than a babywipe every 80 miles or so. This can be used without having to remove the shorts, so is quite easy/discreet to do after a food stop in a quiet layby.
If I forget, and pain does start up (feels like razor blades slashing the area) then a tiny smear of sudocrem on the affected part gives me instant relief. Note I say tiny whereas others up thread have said a handful. All I find is required is enough of the greasy cream to stop the rubbing and allow the cheeks to slide again. Too much and it makes the next babywipe moment messy.
After 200 miles a pair of fresh shorts is LOVELY! Oohh I still get pleasant memories of changing into a new pair of shorts at that point. <happy moment>
Having said that though, when I did my 600 the last 16 hours were in heavy rain and I finished in the same pair of shorts I started in, had forgotten about the wipes, and didn't need any cream. I presume the rainwater was sufficient.