Ours met a similar fate, when we baught it i didnt really know much about the engines foibles and it had done about 10k on the oil that was in it, which looking at the receipts was some cheapy 10w40 semi-synth. Was like warm treacle coming out. I filled the motor with flushing oil and ran it on that for 10mins then drained and it too was jet black.
Since then its done around 25k with regular changes every 5k, but i can tell something in there has suffered because the cam chain rattles constantly, and if the oil level is below around half way and the motor is left idling when warm it starts to lose oil pressure and the lifters start clattering.
It could probably do with a new oil pump and a new cam chain tensioner, but its rolling on 180k miles, so i'm leaning towards just ignoring it, keeping the oil topped up to stop it losing pressure and hoping it continues to rattle on for another 20k.