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Originally Posted by fatlazyless
Engines are just like people. It's not the age, it's the condition. What's bad for engines is lack of use, and obvious things like never changing the oil or not using enough two-stroke oil. The $5.50 synthetics like Amsoil or Mobil One do a better job than the 1.09 Wal-Mart oil, Amen!
Mystery Oil in the gasoline is excellent for four stroke outboards and inboards and no good for two-stroke outboards but does not mix well with synthetic oils. Don't know why but was told that by a small engine repair expert. TechRon carb & injector cleaner, made by Chevron and very pricey, is great for cleaning out injectors, carburetors and gasoline filters and increases mileage.
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Be SURE to check with the manufacturer before using synthetic oil in marine applications! I use Mobile 1 synthetic in everything I own with an engine except for the boat. I did some research on line and found out that Mercruiser (stern drives) very clearly state NOT to use synthetic or multi-weight oil. They do not specify why, but I found this which gives a pretty interesting reason why this is:
More recently, I have noticed that engines with roller lifters used in the pleasure-boat environment have exhibited undesirable wear patterns when synthetic oil is used. Specifically, because the oil is more "slippery," it appears that the rollers are sliding on the cam instead of rolling. This results in flat spots on the rollers and increased wear on the cam lobe. This problem only seems to present itself with lower valve-spring tension applications typically used with hydraulic roller cams. In race applications, with solid roller setups where 250 or more pounds of spring pressure exists when the valve is on the seat, there seems to be enough pressure to keep the roller on the lifter rolling on the cam instead of sliding.
Depending on your climate and the water temperature you typically operate your boat in, the viscosity of your oil should be selected accordingly. I believe that a straight grade SAE 40 oil is the most suitable for the majority of the performance marine engines. SAE 30 would be better for cooler climates. For extreme performance and supercharged engines, SAE 50 is suggested. Most synthetics are a multi-viscosity oil. Fuel dilution of the oil and contamination as a result of increased moisture in the crankcase seems to affect the multi-viscosity oils faster.
I've consulted more than one source to confirm not using synthetic oil or multiweight oils, so when I change the oil in the boat it's always fed SAE 40. This is by far the best explination I've seen so I thought it would be of interest. I did double check the Mercruiser web site and it shows that you can buy a synthetic blend. Normally I don't pay to much attention to manufacturer's recommendations, but when they specifically say not to do something, that tends to get my attention. Running a boat at WOT with a improperly matched prop pitch for the gear ratio can reduce engine life. It all depends on what RPM the engine is running in that condition. I personally very rarely run mine at WOT and not for very long.