Borewash is caused by overfuelling mate -- It happens when the volume of fuel injected into the cylinders is more than can stay as vapour (an analogy is when you spray on deodorant, if you keep spraying it on your oxter it'll turn into liquid and run down your side) On the compression stroke the liquid form of fuel cannot be compressed and forces itself past the pitson rings, taking with it the lubricating oil, which in turn causes ring and cylinder wall damage. If it also makes it's way into the sump, it can thin the oil and lead to bottom end wear. It takes a lot of overfuelling to do this though I'm sure mate, and it's far safer to be a little rich when tuning than lean.