10W-30 or 25W-40

[alumaman]

I run a 2015, 90hp (2.1L, 4S) Mercury Tiller 200 hours per year.

The Merc literature for this motor recommends two different synthetic oils:

10W-30 and 25W - 40.

What are the pros/cons of 10W-30 compared to 25W-40.

Jack

[Hot Runr Guy]

On my 60 4S, I find that the 10W-30 lets my engine crank a little easier on those mornings when there is frost on the dock,,,,,,

HRG

[yarcraft91]

If your outboard is one of those that "makes oil" due to fuel getting into the oil, 25W40 will probably tolerate more fuel dilution.

If both 10W30 and 25W40 are recommended for your outboard, is there anything in the recommendations about the recommended ambient temperature ranges for use of these different oils?

[alumaman]

I have not found any recommendation for 10W-30 or 25W-40 oil specific to ambient temperatures.

I do run the outboard year 'round including water temps to 33 degrees until the water turns hard and outdoor temps down to upper teens when on rivers.

Having said that, I am thinking of just staying with the 10W-30 fully synthetic oil.

Jack.

[Custom Eyes]

From the Merc manual

a - SAE 25W-40 viscosity oil may be used at temperatures above 4 °C (40 °F)
b - SAE 10W-30 viscosity oil is recommended for use in all temperatures.

[yarcraft91]

Originally Posted by Custom Eyes

From the Merc manual

a - SAE 25W-40 viscosity oil may be used at temperatures above 4 °C (40 °F)
b - SAE 10W-30 viscosity oil is recommended for use in all temperatures.

Is that the Mercury manual for the 2015 90 horse? I've learned that the manual recommendations for oil weight vs. temperature depend on which outboard the manual covers. The manual for my 2006 50 horse recommends 25W40 for all temperatures and 10W30 only below 40 degrees F. My manual also recommends against the use of "synthetic oils" (that's a little confusing, isn't it?).

[Custom Eyes]

Originally Posted by yarcraft91

Is that the Mercury manual for the 2015 90 horse? I've learned that the manual recommendations for oil weight vs. temperature depend on which outboard the manual covers. The manual for my 2006 50 horse recommends 25W40 for all temperatures and 10W30 only below 40 degrees F. My manual also recommends against the use of "synthetic oils" (that's a little confusing, isn't it?).

That's for the 75, 90, and 115hp.

The no synthetics is probably because that's around the era where auto and marine manufacturers were still having problems with seals and gaskets deteriorating from people switching back and forth between synthetic and conventional. With older seals and gaskets, PAO synthetic oil actually sucked the plasticizers out of the material and the PAO took it's place. No problem if you stuck with synthetic, but if you switched back to conventional, that oil drew out the PAO in the seals, leaving them with nothing to act as a plasticizer and then they would shrink and crack.

[alumaman]

Custom Eyes -- great explanations. Thank you.

Jack

[yarcraft91]

Originally Posted by Custom Eyes

That's for the 75, 90, and 115hp.

The no synthetics is probably because that's around the era where auto and marine manufacturers were still having problems with seals and gaskets deteriorating from people switching back and forth between synthetic and conventional. With older seals and gaskets, PAO synthetic oil actually sucked the plasticizers out of the material and the PAO took it's place. No problem if you stuck with synthetic, but if you switched back to conventional, that oil drew out the PAO in the seals, leaving them with nothing to act as a plasticizer and then they would shrink and crack.

The "no synthetics" recommendation for my outboard was confusing to me, because the Mercury-recommended oil for my outboard was a Mercury synthetic blend. Mercury has since altered their recommendations to include any NMMA-rated oil for their 4-stroke outboards, regardless of synthetic content, and their synthetic blend oil has become harder to find. Just curious when you believe that problem with synthetic oils was solved, if it truly has been.

[Custom Eyes]

Originally Posted by yarcraft91

The "no synthetics" recommendation for my outboard was confusing to me, because the Mercury-recommended oil for my outboard was a Mercury synthetic blend. Mercury has since altered their recommendations to include any NMMA-rated oil for their 4-stroke outboards, regardless of synthetic content, and their synthetic blend oil has become harder to find. Just curious when you believe that problem with synthetic oils was solved, if it truly has been.

We were doing OEM compatibility testing with failures fairly common as late as 2009-ish. It wouldn't be unheard of for them to have had different seal materials in the two different engines. Likely that the 2-stroke had better seals in it to handle the solvent that is in the 2-stroke oils.