[northern piper]

take a look at the plugs individually, that may provide you with a clue. You could also do a compression test if you have the tools. I'd certainly be doing an oil sample with blackstone labs and they'll undoubtedly shed some light on you problem. I have had similar problems with a different vehicle that was leaking coolant subtly and I ended up using some of the gasket repair gunk to the antifreeze. Solved the problem well. I'm generally not a big fan of the 'quick fix in a bottle' but it may be worth a try.