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Old     (migs)      Join Date: Aug 2006 Location: SF Bay Area       11-05-2007, 9:49 AM Reply   
Called dealer and they said 25/40 Synthetic. Is this what you all use?
Old     (hatepain)      Join Date: Aug 2006       11-05-2007, 10:01 AM Reply   
I use mobile 1 synthetic 5W-30 my dealer said they use a 30 weight so I wound up going with that.
Old     (rallyart)      Join Date: Nov 2006       11-05-2007, 10:38 AM Reply   
I use 5W-40 synthetic. It's never been a problem running a lighter W rating on any engine I've had.
Old     (mobv)      Join Date: Jun 2002       11-05-2007, 10:54 AM Reply   
My Mercruiser Black Scorp said 15W-40. Mercruiser now recommends Syn Blend 15W-40 according to dealer. I have been using Quicksilver (Mercruiser Aftermarket Brand) 15W-40.
Old     (yubasanger)      Join Date: Jul 2007       11-05-2007, 1:56 PM Reply   
I second the Quicksilver 15w-40. As long as the engine is under warranty I suggest using the oil that the engine manufacture recommends. The last thing you want if for Mercury to not warranty due to improper oil. (and they will). The W in 15w-40 stands for WINTER. The first number in oil weight is its viscosity at 0 degrees Fahrenheit (i.e. how fast it will flow). Most of us don't use our boats when it is 0 out side the first number as suggested above is not that important. The second number is its viscosity at 210 degrees. This is the number that we as boaters need to be concerned with if the manufacture calls for 40 weight oil never put in a 30 it will not coat and protect crucial parts like it is supposed to. A 30 and a 40 weight oil don't have the same film thickness. So by using thinner oil you can increase engine wear. As the engine gets older it is recommended to increase the oil weight slightly. This will provide a thicker film on bearings, reduce blow by and oil consumption, it will also help to quiet a valve train that has many hours on it. I would only recommend increasing the weight by one step though. So if your engine calls for 30 go to 40, 40 to 50 and so on. Donít recommend 30 to 50 the oil passages in the crank and pushrods of a motor that was designed for 30 weight will be too small for 50 weight and will starve your motor of oil.

The way oil viscosity is measured is with a Saybolt orifice viscometer, which is basically a cup surrounded by a water bath which keeps the temperature of the oil sample constant. There is a plugged opening in the bottom of the cup, which is opened when the oil is at the proper temperature. The oil is captured in a 60ml flask and the amount of time required to fill the flask is recorded.

A SAE30 oil takes 58-70 seconds to fill an SAE40 oil takes 70-85 seconds, and
an SAE50 oil takes 85-110 seconds.

There is also a test for low-temperature viscosity that is designated by a "W" suffix on the SAE viscosity rating.

A SAE5W oil takes less than 6000 seconds to fill
An SAE10W oil takes 6000-12000 seconds, and
an SAE20W takes 12000-48000 seconds.


I'd assume that the winter oil testing leaves the test engineers with plenty of time to relax while waiting for the test to conclude! Ah, the life of a petroleum engineer!

A "multiviscosity" oil is simply an oil that has been rated using both tests. Nothing magic about it, except that sometimes the composition of the oil has been tweaked to flatten the viscosity-temperature curve.

Note that a SAE30 oil and a SAE30W oil are not the same thing, as they are measured using different tests.

Happy boating.
Old     (olskooltige)      Join Date: Mar 2007       11-05-2007, 2:17 PM Reply   
A better definition is to think of multi vis as a weight that thins no less than another. Example:

10W-30
It is a 10 weight oil that is no thinner than a 30W oil at operating temp.

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