Everyone has a friend, or a friend of a friend, or somebody who has destroyed just about EVERY component in every RC car becasue of something seemingly harmless. I find that its best to only speak of experiences you yourself have actually had. So that way you know all the facts about it. He may be a friend, but you can't follow him home and see what he's doing to his engine there.
I ran a OFNA .15 engine for years using WD-40 as my only afterrun lube. And the thing still has excellent compression. Now I use Hobbico's after run oil. Which as I understand it is the same as every other 'red colored' after run oil.
But like someone said before, if blew a rod or whatever in his engine, he needed more than wd-40 to do it!