You wonder why there is so much confusion on oils.

A 25w40 IS A MULTI-VISCOSITY!!!!!!!

Who told you that an XX w XX oil IS NOT MULTI-VISCOSITY?

It has no bearing on the viscosity whether the oil is blended or not.

And the oils you suggested...guess what, ALL MULTI-VISCOSITY.

Ken

15w50, which by the way is a multi-viscosity oil just like the 20w50.

Ken

15w40

I run straight 40w. I remember Bob Teague recommending straight weight oils in a tech column a few years back. IIRC, it had something to do with multi-grades not handling moisture as well as straight grades, and who starts there engines and just takes off with them cold anyway?

And Teague Runs a 15w-50 in his boat now, whats that tell ya?!! Straight weight oils are OLD TECHNOLOGY! It'd be like buying a brand new Vette and putting an AM radio in it.

15/40 is fine and I don't remember correctly but that' could be what's recommended. Mercury oil is a 15/40. Boat engines need a higher viscosity rating than car engines for a few different reasons. I'd run a 15/40 or even a straight 40W if you boat in warm weather. I run straight 50W in my 800's.

Easy, Killer. Merc states (stated?) that even though they mark their oil as a 25W40 it is actually a blend of 25W and 40W oils. Therefore, not a multi-viscosity oil. If you look back through the old Hydrocruiser oil thread I believe someone did a lab analysis on Merc's oil and it stated what straight weight it actually equates out to (30 something). I did not just make this stuff up. I know what a multi-viscosity oil is. That is why I stated the caveat about the Merc oil. Unless something has changed, they claim it is a straight weight oil even though it is marked as a multi-viscosity oil. If someone knows more about the Merc oil, please chime in. I know I read it somewhere at some point (maybe the bottle?). If I misunderstood Merc's description of the oil then I apologize for the misinformation.

Edit: here is the latest description of the oil. Different than what I remember from the past. Merc states it is a multi-viscosity. I will look through the old oil thread to see if I can find information to back what I believe Merc used to market the oil as.

http://www.mercurymarine.com/parts-a...ercruiser-oil/

Mobil 1 is treated as a multi-visc but in reality it's not blended as a multi-visc oil like most other brands. It's classified with that rating because it has the ability to flow in cold weather like a 15W but also has the ability to protect at high temp/load like a 50W.

That's my understanding at least.

I was just stating what I run and one of the reasons why I do what I do. My 454 mag has been running on straight wt oil for 21 years, and I highly doubt that multi-grade oil would make it last any longer. Multi-grade oils are for people that start their engines and are too busy to warm them up before running them. Yes, multi-grade oils are newer technology, but not necessarily better technology, they are for different applications. Brad Penn and Royal Purple make and sell SYNTHETIC STRAIGHT WT oils.

Here is what I was referring to from the old oil thread. As you can see, Hydro provides info that backs what I remember reading on the back of the Merc 25W40 bottles a few years back. It was a blend of straight weight oils, not a multi-viscosity (though labled as such). Confusing to the consumer. Obviously from the info on their site they have since changed the oil to be a true multi-viscosity. Sorry for any confusion I caused for not staying up to date with Merc's oil. I have not used it since early 2005 (to break the motor in). See the first couple posts.

http://www.offshoreonly.com/forums/g...cation-72.html