I use what it says to use in the service manual. The books says for my 7.4 L engines to use strait 40 weight. When I got the boat at 650 hours the owner was using 15 40. I switch as the book said to use 40 weight. I like shell but it is hard to find in Canada so I use name brand and all is well so far. Engines have 2750 hours on them and they do not burn oil and the power is the same as it was when we first got the boat. I change the oil every 100 to 130 hours. Oil dirty at 100 hours.

 

Thanks. That helps.

 

There are other oils in the same manual from what I remember based on ambient temperature. The 15 40 would be nicer as the 40 is thicker even when hot and it takes half an hour to full drain an engine. My engines have a 1/4" ball valve in the oil pan I drain the oil through into 1.75L plastic whisky bottles. The 40 I use says for diesel engines. Never seen any that says for gas engines.
My engines are the 330HP model.
Turn the blower on and open all the hatches as it is hot around the engines. Know where your fire extinguishers are when you change the oil. Never heard of anyone ever having a fire changing oil.
It is a simple job no more difficult than on a car. It is a 1 beer job for per engine watching the oil drain out.

 

This from a Mercruiser manual, it's a few years old, but the oil recommendation has not changed:

To help obtain optimum engine performance and to provide maximum protection, we
strongly recommend the use of Quicksilver 4-Cycle 25W-40 Marine Engine Oil. This oil is
a special blend of 25-weight and 40-weight oils for marine engines. If not available, a good
grade, straight weight, detergent automotive oil of correct viscosity, with an API
classification of SH,CF/CF-2, may be used.
In those areas where Quicksilver 4-Cycle 25W-40 Marine Engine Oil or a recommended
straight weight oil are not available, a multiviscosity 20W-40 (SH, CF/CF-2) or, as a second
but less preferable choice, 20W-50, with API service ratings of SH, CF/CF-2 may be used.
IMPORTANT: The use of non-detergent oils, multi-viscosity oils (other than Quicksilver
25W-40 or a good quality 20W-40 or 20W-50), synthetic oils, low quality oils or oils
that contain solid additives are specifically not recommended.



Personally I just use the Quicksilver oil, always have. If I were to go with another brand, it would probably be Rotella 15w 40. Walmart sells the Quicksilver, so for my once a year oil change that's the easy route. Bottom line there is no wrong brand as long as it meets the specifications laid out in the Mercruiser manual - you do that and change it on a reasonable interval, oil is the last thing that will cause problems in a marine engine. The Correct Craft in my signature ran 2500hrs using Valvoline straight 40w, not a hint of using oil.

 

Northern thanks. Whisky bottle?! Nice use. I have several on the boat. I'll work on emptying them.

 

Right now I can't find my manual. Not sure what I did or somebody did. Thanks for the help. Plan to do in the next day or two.

 

The oil I use is not synthetic.
I use 10 30 in the Quicksilver generator. That oil is changed once a year or every 100 hours what ever is first.
I am no mechanic but I did run the maintenance department for a large pipeline system. From that I found that being consistence and following what the manufacture recommends was what made things last.
We are 800 miles from the boat and will not see it until spring. When we go out I will scan the page and post it. Main thing is to change it and the oil filter every 100 hours or so and do not put the boat away for the winter with old oil in it.

 

Just changed 2 496 gassers (05 360DA ) to Mobil 1 15-50 full synthetic. Pressure is still spot on .. NO consumption. Runs smoother & quieter ( to me ) ....

That will be what they have until after I dont own anymore

 

16 years of using the Mercuiser Oil. It has been fine. A quality oil is always going to work as listed above. I just don't have time to think about it. So I just go with the simplest choice.