View Full Version : What motor oil for a 4.3L mercrusier?

10-08-2005, 08:06 PM
Mercrusier says to use a single weight oil like 30w or 40w, and not to use a multi-grade oil. What I would really like to do it to use a 10w-40 or 10w-30 synthetic, as I think that would be the bomb for a boat engine. What do you guys think? What do you in/out guys use in your engines?

On edit: not sure if this is important, but the boat/engine is a 1987 Seaswirl, 4.3L Mercrusier (chevy basically) with a Cobra outdrive.


10-09-2005, 05:11 PM
Hi, I have a 1989 Welcraft with a 4.3 O.M.C and I use castrol 20-50 synthetic.

10-09-2005, 06:09 PM
Hi, I have a 1989 Welcraft with a 4.3 O.M.C and I use castrol 20-50 synthetic.

And you have had no adverse problems related to the oil? How long have you been using it?


10-10-2005, 05:28 AM
8)Norcalfishslayer I have a 7.4 bravo merc cruiser and I use a top of the line fram oil filter and quaker state high rev synthetic It makes that engine purr at wot you can barly hear it and it helped on fuel economy. I also run it in my jet sled great stuff. ;) 8)

10-10-2005, 09:09 AM
There is a lot of misinformation about the right oil to use for inboard gasoline boat engines. Some of it is propagated by the engine manufacturers in order to get you to use their own brand of oil. Remember that all of these engines are just marinized automotive engines. The 4.3 Mercruiser is the same engine used in GM cars and trucks. Marinization just uses a different cooling and exhaust system and spark arrested electrical components. All of the other other components are the same as the automotive engine.

But there are differences in how a boat engine is used vs a car or truck and that is where the different oil requirements come in. Boat engines are operated at much higher rpm and load conditions than a car or truck engine. So the properties of a good marine oil that gives maximum protection under those conditions are similar to a "racing oil" used for competition rather than on the road cars or trucks. However, racing oils are not designed to hold up well under conditions of infrequent use and high humidity. The important properties for a marine oil are:

1. Sufficient viscosity to provide protection during acceleration and operation at high rpm.

2. Resistance to mechanical shear. During operation under high speed and load, the oil molecules can align with each other under extreme pressure and the viscosity can dramatically decrease.

3. Anti-foam additives. High rpm operation can result in aeration of the oil and reduced protection.

4. High levels anti-wear additives. These are compounds that provide protection against metal to metal contact during startup and under high load conditions where vibration can break down the oil film. The most commonly used anti-wear additive is a zinc and phosphorous containing compound that is now found only at reduced levels in automotive oils because it poisons catalytic converters in cars. Boats don't use cats.

5. High levels of anti-corrosion additives. Boat engines typically sit for long periods of time. As acids accumulate in the engine oil during operation, they must be neutralized by additives or metal corrosion will be accelerated. The longer a boat sits without running, the more the oil film will drain off the engine parts and corrosion becomes more of an issue. This can be minimized but not eliminated by proper oil additives.

So what does this all mean? Unless you operate you engine very gently and change the oil at least twice per season, automotive oils are not sufficient for good protection. Multi-viscosity engine oils such as 10W-30 are really 10 weight oils with a small amount of a viscosity improver added to keep the oil from thinning below a 30 weight at 212 degrees. The problem is that marine engine parts can get hotter than that at high load and rpm. Plus the viscosity improvers tend to shear at high pressure and temperature. That's why marine engine manufactures recommend straight weight oils or their own brands of multi-grade (like Merc 25-40) that do not contain any viscosity improvers. So other than using the engine branded oils, which by the way are all very good oils but cost a lot of money, what kind of oil should be used? Here is the dirty little secret. Look at the rating on the bottles of any marine oil. You will see that they contain a bunch of letters like SJ or SI. But the key is that they always also have a CH-4 rating. That means that they are certified for use in diesel engines. They use a standard diesel engine oil additive package that contains higher levels of anti-wear, anti-corrosion, and anti-foam agents than automotive oils. Combined with a shear resistant base oil, you now have a marine oil. Oh yeah, diesel oils already contain a shear resistant base oil designed to hold up under extreme pressure and load.

So there you have it. Use a diesel oil and you have everything you need to protect a marine engine for a whole lot less money than you'll pay for a branded boat oil. Back in Florida, everybody used 15W-40 diesel oils in big block gas engines and they held up for thousands of hours. There are straight 30 weight diesel oils available that would also be fine. Any synthetic diesel oil should also work great. I know guys who ran 15W-40 Amsoil AME in their inboards with great results.

10-10-2005, 05:35 PM
;Dthats throwin the book at him Doc, always informative and complete. ;D 8)

10-10-2005, 06:03 PM
Wow Doc, excelent post (like always!) ;D I'll have to go out and see if the synthetic oil that I bought has the diesel rating. I have used Rotella T synthetic in my diesel truck before. Maybe I'll take this stuff back and get some of it. Again, thanks for the info...


10-10-2005, 06:36 PM
Thanks for the great post Doc, Lots of info.