Came back from boating past weekend and as always, plenty of soot on the transom from the 496mag ho. Never been a problem, just wash and add more oil. Motor has 395 hrs on it now, not run hard. I noticed I had a few oil specs on the transom though along with the soot (was thinking maybe a problem with the oil cooler?). I checked all the plugs, they were all fine, all gapped fine. I changed them all and also changed the oil (no water in oil), fired the boat back up and all is fine. I think I had this happen a little once before. Someone recommended a compression check but then someone else said "why", you can't repair that engine anyway. Anyone have a similar problem ever? Thankfully boat is running fine, no alarms, just curious. Thank You.

You don't say what kind of oil you're using. If not already, try running Mobil 1 15W50. My 300+ hour 496HO's were losing 1/2 quart every outing when I ran 25W40. Since the switch, I've put 20 hours on the engines and haven't used a drop of oil.

I have always run the Mercury 24w/40. May be time to change! Is it a viscocity issue or just Merc oil? Thanks for the reply!

I'm not certain, but my theory is that Mercury builds the 496 with a very loose ring tolerance for longevity. At idle, there's probably not enough seal to keep the thinner oil from sneaking by. The thicker oil stays put. I don't believe it's an oil brand issue.