Very dark engine oil - why?

I owned my boat for three years and changed the engine oil by myself every year. I noticed that every year the oil was very dark brown, basically black. There is no any milkyness or debris in the oil. It is just very, very dark. Is this normal?

I feel this is very different from the car oil: After 5000 miles the oil in my Toyota SUV looks brown, but not nearly as dark as the oil I suck out from the boat engine. The boat is used very little. I make may be 20 fishing trips and 10 fun trips per season. On some of the trips I troll at the minimum throttle for few hours.

The oil I put in is Quicksilver Synthetic Blend. The oil filter is also Quicksilver. The engine is never overheated (according to the gage). The overall life of the engine is only about 500 hours. The boat is kept on a trailer inland, always covered, so the chance of rainwater getting into the oil is zero. I do fogging in the fall. Engine: 1993 4.3LX/V6 Alpha One Thunderbolt Ignition, serial number OF0244564. Boat is fiberglass, 20-feet Searay.

So, if this is abnormal, what may be the reason?

Re: Very dark engine oil - why?

Are you using purple dyed gas? It is a common thing for the dye in fuel to affect the colour of the oil. Have you had the valve covers off? Perhaps the previous owner was a little lax in his servicing and there is some crud built up inside the motor and your fresh oil is slowly dissolving it.

Re: Very dark engine oil - why?

babikov - 20 fishing trips and 10 fun trips can add up to a lot of hours. Do you have an idea of how many hours you log? Also, what is the clarity of the oil when you put it in? Some are quite clear and some are brown when you start. Also, what is normal temperature when you run?

With no idea of the answers to these questions, my guess is that you're seeing normal deterioration/oxidation. It is a guess, mind you, but I'm thinking that you will not find anything amiss.

Re: Very dark engine oil - why?

If the answer is worth $25, send a sample of the dark oil to the lab:

http://www.blackstone-labs.com/marine.php

Your welcome!

Re: Very dark engine oil - why?

Dark is fine. It means the oil is suspending the dirt that it should. It means you have a quality, high-detergent oil.

Oil manufacturers, who want you to change your oil to their benefit even say it is proper to see dark oil. Thin oil, or brownish oil, has always been worse in my book. It means the detergent additives aren't working or the dirt suspending properties aren't working, or the engine is way too rich or their is blow-by (combustion byproducts getting into crankcase -- a sign of a tired engine). After regular use, light or brownish could mean excess moisture or flat out water intrusion. When I pull a dipstick, I want dark, dark, dark brown or black oil, unless it JUST came out of the bottle.

Just ONE example, from Valvoline:

Does dark oil means it's time to change it?

Not necessarily. Dark oil means that the oil is using the detergents added to the formulation to clean the inside of the engine. If an oil is dark, that doesn't mean that the additives are not working, it just means that the oil is doing its job to clean internally.

Re: Very dark engine oil - why?

Check the oil level frequently during the next interval. Pull the dipstick and smell it. If it smells like gasoline your long hours of trolling can dilute the oil. Probably not enough to raise the level on the dip stick but that could be part of the problem.

Re: Very dark engine oil - why?

Thanks for all tips. I checked and it appears that I was making roughly 50 hours per year during last three years. True that this is not as little as I thought. So, may be indeed I see a normal process. The oil gets dark not immediately, but progressively, as season goes. What is "valve covers off"?

I will keep cheking as it was suggested above. Thanks.

Re: Very dark engine oil - why?

As suggested pull the valve covers off or at least open the the oil filler cap and look into the valve train. If you see a bunch of sludge laying inside the head that will be what is turning your oil black as it is disolving that sludge slowly. My I/O is extremely clean inside, no sign of any sludge and my Mercruiser oil stays reasonably clean, just getting dark at the end of the season when time for a change. My 50 yrs experience has shown that oil darkens more quickly if the engine has some sludge buildup in it. Even my car (175000 mi) with regular oil changes has little to no sludge in the engine and it takes just about all 3000 miles before the oil gets dark.

Airshot