my mums other half bought an oil treatment off one of the shopping channels for his daewoo which is meant to line the bores and help the oil cling to the vital parts.
I advised him against adding anything to engine oil, if it was meant to be in there it would come with it.

however he didnt listen, he added it to the oil.
a few months later he phoned me saying the car was popping and backfiring.
after having a look at it and checking the normal things I removed the cam cover to find the cam lobes at the cambelt end had worn down so much that they were completely round so the valve werent opening.
I guess the stuff he added to oil must have thinkn'd the oil so much it starved the cam of oil.
expensive lesson to learn.
not sure what the name of the additive he used but ill ask him sunday when im round there