My Northstar did something strange yesterday. I slowed down to go through a toll booth, when I left the toll booth there was a cloud of blue smoke left behind. I drove a bit further and there was no oil smoke. I slowed to exit the highway and another big puff of smoke was behind me. I drove through town and no blue smoke. Now I know the engine is using some oil, about a liter every thousand miles, but I have never seen a cloud following me. The engine runs strong, no signs of troubles as of yet. I've only had this Deville for a couple of months and it works great. Considering there is 547,000 Kms, Yes I said 547,000 Kms, on it the ride and handling are good. I don't know if the engine is original or not but it runs smooth and is clean. I can't see any sign of oil smoke at idle either. If it does actually have 547,000 Kms on it I think it deserves a drink of oil. There doesn't seem to be any substantial amount of oil leaking, no drops under it after it sits. The engine was spark knocking under light load before I noticed the puffs of blue smoke. I figured it was the 89 octane I put in it instead of the 91 or 93 that I usually use. I added a bottle of octane booster and the knock went away. When it knocked the knock would go away if I slowed or sped up slightly. So, on the 50 mile drive home, "I drove it like I stole it" and didn't see any oil smoke following me. I would think that if the engine had internal trouble it would smoke all the time. Any ideas as to what might have caused it to smoke in the first place.