I write the Sterndrive Questions and Answers for FAMILY AND PERFORMANCE BOATING MAGAZINE and would like to know what you guys think of this answer that will be going into the March 2003 issue.
Sincerely
Dennis Moore

Question,
I get confused on the issue of oil viscosity for a marine engine, Mercruiser oil is 25W40, some racing oils are 20W50 and the traditional oils are 10W40. What about single viscosity 30 or 40 weight oil? Is there a simple way to explain the differences between single or multi viscosity oil? Which should I use in a marine engine?

Answer,
Actually there is a simple way to explain the differences between a single or multi viscosity oil that seems to get lost in every discussion about engine oil. The simple fact is this, multiviscosity oils serve only one purpose, they are designed to flow easily at low temperatures. If the oil is excessively thick (high viscosity) the starter motor will not be able to spin the crankshaft fast enough to start the engine in cold weather. If you start your marine engine in temperatures lower than 40 degrees Fahrenheit then you may need multiviscosity oil. As most boaters do not use their boats at such low temperatures multiviscosity oil is simply not necessary.
There is a minor drawback to using multiviscosity oils. Using the most common multiviscosity oil as an example, 10W40, we know that oil refiners start with a 10 weight oil for easy flow at cold temperatures and then they improve the oil viscosity with an additive called viscosity improvers so that the oil will thicken to 40 weight when it becomes hot. Viscosity improvers can break down if the oil is overheated leaving oil with a viscosity of only 10 weight inside your engine. Sometimes this is noticed by a drop in oil pressure on your gauge. Oil refiners start with 40 weight oil when manufacturing single viscosity 40 weight motor oil and use no viscosity improvers. Without viscosity improvers to break down the oil will always remain at 40 weight viscosity.
Mercruiser recommends there own 25W40 oil for marine engines but it is not really a multiviscosity oil in the normal sense. Mercruiser oil does not contain viscosity improvers, it is simply a blend of 25 weight and 40 weight oils.

This was explained to me over 20 years ago exactely as you wrote it. I think most people don't realize the stress a marine engine is under. Always under load, there's no downhill on the water. Also most marine engines operate at a very high RPM, no 5 speed overdrive transmissions. I think oil coolers make a big difference. The old Mark 4 330's didn't have oil coolers and really needed the straight 40 wt.

Thanks,
Anyone else?

Thanks Dennis

I use the valvoline straight 40 weight racing oil all summer, then put in the 10-40 when I winterrize. Swap it back out in the spring. Although after reading this, I think I'll buck up for the Merc 25/40 mix instead for the winter.

If it's below 40 degrees, it's no longer called pleasure boating!

I tried the valvoline 50 racing oil, but I seemed to notice a slight oil pressure decrease. Maybe it was my imagination, but the straight 40 seems to be about right. (stock 454mag mpi)


Keep up the good work Dennis!

Thanks,
Anyone else?

Dennis,

I read somewhere (can't remember where), that true synthetics do NOT use viscosity improvers except in the very wide spread weights like 15W-50. Supposedly, anything that is 15W-40 or "tighter" does not use improvers.

I try to find this info again, but you may already know something about it.

I cannot find it, Dennis. But I read that true synthetics in 15W-40 form, for example, start cold as 15W oil but maintain the viscosity of a straight 40W oil at high temperatures, but WITHOUT viscosity improvers. 15W-50 syn oils do require improvers.

Sorry I can't find the source to back it up.

Rick,
I read that on the Redline web site. I better make the distinction between mineral based oils and synthetics.
Thanks much!
Dennis

I use a straight 40 weight mineral based oil. I have oil thermostats and large oil coolers, and I change my oil after about every 15 hours. I was told by some engine builders that when using roller cams (I have hydraulic rollers) that a synthetic oil is too slick for the roller lifters to function properly as they run across the cam lobes. Fact or myth I am not sure, but that's what I use.