what is the problem when you use 0-w oil?

Breaks down too quick at heat i think. Low weight oil is good for very cold temps b/c it flows well when cold. But a hard running marine engine should have something heavier. I use 20w-50. Use that or 15w-40, 15w-50, or 20w-40.

I didn't find him condescending. I didn't know the answer. Found it informative. Thanks!

Here's my 2cents.. A lot of you already know this, but modern automotive oil has had a lot of the good ingredients removed because of vehicle emmisions standards. Apparently the stuff that helps shear protection also plugs catalytic converters. Mercury has just sent out a service bulletin recommending only using their 20w 40 full synthetic oil for ALL gasoline sterndrives. I can send anyone a copy if they want.

Alternatively if you want to run a conventional oil, any good high speed diesel oil will give you the best protection, especially for the guys with flat tappet cams. Shell Rotella T, Ursa Super Plus, and Delo 400, all 15w40 tested the highest with Pennzoil 20w50 for gasoline engines being the absolute worst. These were indepent tests so don't sue me pennzoil :party-smiley-004:

oh one more thing.. You can buy GM EOS (engine oil supplement) from any dealer and add it to whatever's already in your motor and you'll be fine.. Alex

You are refering to the reduced levels of ZDDP in automotive oils.

Merc now has a full synthetic? I know they have a semi-synthetic 25W40.

Exactly! According to the bulletin they do.. Haven't seen it on the shelves yet..

Following the lead of the auto industry............
 

..........I guess it's ok to use a synthetic, but I wouldn,t use this one.
Ken

Of course Mercury is going to "recommend" only use their 25w-40, because they WANT TO SELL IT!!!! You can use any oil out there, as long as it meets or exceeds the specifications required. Also, you should never have to add any additives to your oil. IF THEY WERE MEANT TO BE IN THERE, THEY WOULD BE IN THERE FROM THE START. They can actually work against each other and cause premature breakdown and shearing, therefore losing protection. If you want to the best protection either use AMSOIL or Mobil 1. They are tested and rated as the number 1 and number 2 oil out there. IF anyone out there thinks Mercury makes their own oil, they are HIGH! They bid it out to the cheapest manufacturer and have it made for them. So in other words, its probably junk!

Oh yeah, and to anwser the 0w question, IT DOESN'T MATTER whether its a 0w, 5w,
10w, 15w or 20w-40 or 50, the second number is what you should be concerned with, that is protection rating when the oil is hot! The first number basically tells you at how cold of a temp it will still pour. So actually, the lower the first number the better. That is why Hendrick Motor Sports uses 0w-30 for qualifing

I wasn't suggesting that everyone should rush out and stock up on mercury oil. Simply pointing out that Merc has changed their stance on the use of it. Rather then getting into a big discussion about low tension rings, bearing clearance, etc..
I think the basis of this thread was wether or not it was ok to run synthetic oil and what type to run, specifically in regards to the 496HO. Ray hit it right on the head.. :D

If you read the technote that 07Dominatrix supplied, it applies to Merc engines equiped with cat converters (aka CATS), which I did not know they were doing. The most recent API SL, SM specs are reducing certain additives (zinc and phosphorus) because those items harm cats. I have spoken to tech support at both Amsoil and Royal Purple, and they indicate that newer, non-syn oils meeting those specs are actually not as good for your engine as previous specs- its a trade off of engine protection for emissions equipement longevity.
The issue with low vis oils (aka 0w, 5w etc.) is more for non-syn oils and how much viscosity index (VI) improvers are used. Lets take 5w30 oil as an example. Most non-syn oils start life as as a 5w oil, and a VI improver is added to make this oil act like a 30 at higher temps. The problem with this is that this additive tends to wear out quickly. With good sythetic oils, no vis improver is required to have the oil behave like a 5w at 0F and a 30 at high temps.
The problem with something like a 0w40 is that it can be very hard to tell if, and how much, VI improvers are used. If a lot of VI is used, this oil could 'wear out' (from a vi perspective) quickly under high engine loads.
As for race car drivers using light oils, this is true- but they do this because it gains a few HP that would otherwise be lost pumping a higher weight oil. Again, they are trading engine wear for HP, which for many racers is acceptable if not a downright nessecity.
For a high performance boat engine, you would be best served in using a good synthetic 15w-40, 15w-50 or 20w-50 oil. I had a 454 Mag engine go 1600 hours on a combination of Mobil 1 15w-50 and Amsoil 20w-50 (about a 50/50 split). After 1600 hours of use, the engine still had great readings for compression and leakdown, so much that I was able to sell the engine to somebody that needed a running motor for a swap.
Also, a word of warning- there are actually some crap oils out there. They meet the min specs but not much more. Castrol Syntec 5w-50 was widely known as a crappy oil sold at a premium price. It was not a true synthetic (perhaps a Group 4) and it required lots of VI improvers to meet the 50 number. And all Mobil 1's are not the same- The M1 labeled Extended Protection (or similar terms) is a better oil than the regular M1, hence its 1-2 dollar premium price.

My brother is a mechanical engineer, and a good friend of his works for GM (or maybe Delphi now, not sure). Anyhow, I spoke to him at length about oils at a party, and he told me that the 454 7.4l L29 engine I have in my boat is identical to the engine that went into GM trucks, with the exception of cam, starter, possibly freeze plugs and a few other things. What we know as the long block (excepting the cam) is is identical to the truck motor. For the truck, 5w-30 or 10w-30 is recommended. For the same engine from Merc, they recommended 30w or 40w oil in the owners manual. Since engines in boats work so much harder than cars and trucks, the reduced wear at cold start advantage of the lighter oil was not worth the accelerated wear under load, especially if a marginal oil was used.

My recommendation is that if you are the average powerboater, you would be best served to run a heaver mulitweight synth oil (15w40 to 20w50), and I would not run a straight weight 40+ unless I was racing, and I would not run less than 15w unless I was running in cold temps (under 40 F).