time to change

[floater180]

what kind of oil do you use in your boat motor? synthetic or crude and what brand?

[Babyboomer]

I use Mercruise Synethic Blend

[Vuja De]

Me too on the Merc Synthetic

[Dream 'Inn]

Merc 25W-40.

[PartsMan]

Merc 25W-40.

+1

[Babyboomer]

Synthetic Blend is 25W40

[Rinker342]

Me too regarding the Merc synthetic blend.

[alswagg]

We have been using synthetic blended 15W40 oils for the last 6 season's.  No problems at all.  I do however add Lucas additive to engines with flat tappet cams, for the additional ZDPP anti-friction.  I run the same oil in all of our personal vehicles also.  This oil is recomended for all Indmar and PCM engines, plus Mercruiser if running in our climate.  VOlVO is the exception, they recommend a straight 30wSynthetic, hard to come by.  The engines are all built by GM and the internals are exactly the same.  It is no secret that I do not care for Volvo, I only service less than 10 a season.
Al

[Capt Ron]

I used only the Merc oil same as Boomer - Synthetic blend 25 - 40W

[boataway]

  The engines are all built by GM and the internals are exactly the same.  It is no secret that I do not care for Volvo, I only service less than 10 a season.

It was a secret to me! Mercs and Volvos are exactly the same on the inside? crazy! i always wondered why the displacements were so coincidently similar.

so if they're the same, why do you like the mercs over volvos?

[doubleogt]

Merc 25W-40

[Capt Ron]

Only use the oil that the manufacturer recommends for your engine. The viscosity numbers are only part of the lube equation. The API is also just important. Additives that are in marine grade oils are NOT the same as automotive oils. Marine engines are operated in the most harsh conditions and need these additives to lube correctly. I know that some people decide to go with other oils and with some perceived success. However, this may only work in some climates and not at all in others. Stick to the manufacturers recommended oil, they are the experts.

[faye]

I also use synthetic oil, i guess that is much better.

[fiestavee02]

mobil 1 15w 50 synthetic.
i am willing to make a small wager that mercruiser does not make its own oil.  even if they did, how would their r&d budget compare to a say mobil oil or any other oil specific company.  i would venture to guess, infintessimal, compared to mobil, exxon mobil that is, which happens to be one of the largest companies in the world.
i've heard the usual story about marine oil additives.  i'm not convinced.  has anyone read any hard marine oil performance comparative data? also, mercruiser oil is not pure synthetic, only a blend.  synthetic oil is generally superior in all facets of oil performance criteria vs. a blend.
mobil 1 is proven and tested over millions and millions of miles of extreme conditiions, and proven in nearly every form of automotive racing, including formula 1, the pinnacle of racing technology.  i keep hearing that oh marine engines have it hard compared to auto engines running at a constant lets say 3500 rpm.  mobil 1 is used ubiquoustly among formula 1 teams, where engines race at more than 16,000 rpm and at somewhat lower rpms, lemans race cars race for 24 hrs. flatout at over 250 mph. 
i just don't see that a boat engine manufacturer has anywhere near the r&d and budget capability that is presumably necessary in constantly developing and improving their oil compared to mobil.   when is the last time mercruiser announced a new oil.  i don't know for sure, but mobil and other auto oil companies, seem like they are launching new oil formulas, every 6-12 months.
mobil 1 in head to head testing vs other auto oils, simply is superior in the various oil performance parameters, i.e., flow rate, temp. resistance before break down, viscosity, etc., although recently, thanks to a rbo member, i looked at some data that suggest amsoil may also make a very high quality synthetic oil.
just my 2cents. 
david

[Herm]

Amsoil is the makers of the 30 weight that is used in the volvo motors.