I'm running the Mercury 25w40 four stroke oil, and Mercury oil filter. The idea that some black oil trapped in the oil cooler and other area's in the engine has crossed my mind. I would think that after changing the oil as often as I do would eventually clean things up? I have also had my ECU changed twice by Mercury because I had issues with soot on the transom...it was bad. Now it seems to be not as bad? I had the new ECU installed at the end of the boating season, so I'm going to have to see how it performs in the spring.
Lets keep the idea's coming....

We have never seen the oil temps higher than 200

TEST the oil before and after it turns black and check the results, the blackening of the oil may be nothing but the characteristic of the oil. The TBN number from the test and the readouts on various metals and fuels etc. if in the oil will tell you the real condition of the oil and the results will tell for sure if the oil has contamination that's causing the black color and not just a color change with no real oil problems.
Everything else is a "WAGMAR" (wild ass guess made at random).

Regards,
Ray @ Raylar

:party-smiley-004:Mr. Cozzi touched on the issue of engine tune-up, but perhaps a little more engine status detail will help:
plugs - wet/dry, color, condition, uniformity across all 8?
compression?
oil consumption rate?
fuel consumption - any metrics to compare with?
any engine mods you haven't mentioned?
General overall performance.:cool-smiley-011:

Here are some of my recent results. The oil cooler is mounted at an angle and was designed not to trap oil for obvious reasons. This is 2 different reports (notice sample dates) and the rest of docs where it even tells you how to read charts and what each factor means. Further, it says what this particular lab claims to be "normal". I was not completely satisfied with these answers. I would think that Ray knows as much about the 496 as anyone on OSO. btw, if anyone has an extra Raylar kit laying around, I'd be happy to use it for you....... and let you know if it cleans up my oil!!

this post is only because i ran out of room to attach the 4th page...

with all due respect, lots of people including Blackstone keep falling back on that comment about "residual oil" left in system. I recently saw one of these motors torn down due to high heat and lack of lubrication. I inspected the depth of oil pan and oil removal hardware carefully. It's an easy blame when you like to type short answers but it's not the right one.

First - there is NO difference which hole you suck the oil out off... It DOES matter however, whether or not the boat is reasonably level during removal of oil. If it's tilted up - both drains are located in the middle of the pan leaving much oil in lower area of pan in this instance.

Second - Yes, there is always some residual oil left in engine and a little bit in pan but none of this is why my oil is getting dark fast. There's quite a bit of residual oil left in my duramax but every time i change it, it stays gold for at least 2,500 miles. That's huge for a diesel in my opinion. There is another cause. Arm yourself 496 owners; an attack is coming.

:ernaehrung004:
I agree with Reckless32 - dirty ECU and soot on the transom indicate ring blow-by, burning the oil on the cylinder wall to cinders causing the black oil and air-borne residue.

[QUOTE=monstaaa;another thing to check is the engine oil cooler. on many 496's i have serviced i have found restricted oil coolers, loaded with seaweed or eel grass and the like. this will also help in turning he oil black and will not show up on the smart craft monitor or the oil temp guage.[/QUOTE]

but... if the oil cooler was restricted with seaweed or eel grass, the oil temp guage would tell you because oil temp would go up due to the coolers less ability to cool unless there was a trivial amount of debris in there which would not show up on temp guage cause the oil was running normal temp.

I'll throw another variable into this...

on top of seemingly rapid oil discolorization (no sooty transom, my engines run great and make good power)...

running 25/50 M1, The crankcase fumes are very strong in the cockpit at cruising speeds until the oil gets 4-5 running hours on it...

and no I'm not kidding.
I have done a few oil changes in 230 hrs and it's always the same! At least with M1!