Yanmar 3GM30F Oil

[dmksails]

I last had my oil changed by a mechanic who used Delco 400 SAE-30 oil. I'm getting ready to change the oil by myself. The book calls for a 30 or 40 weight oil. I've also seen that most people online are using 10W-40 oil.

I'm sailing in Puget Sound where the summer temps stay mostly in the 70's. Is there any benefit to using 10W-40?

[a64pilot]

More likely 15W-40.
It’s fine, some swear by it saying that it flows better in both cold and hot temps. It is likely by a wide margin the most used oil.
You can even run pure synthetic 5W-40 Shell Rotella T6, an excellent oil.
But we don’t operate in either extremes of hot and cold.
I used to run the T6, then finally came to the realization that it just wasn’t necessary, although I usually always run the best oil there is, heck with the price.

I now run straight 30W, and think it’s just fine. I still change my oil every 100 hours though, I’m not a believer in running to past that.

So, 15W-40 is fine, so is straight 30W.
Straight weight oil doesn’t usually break down as fast as multi viscosity oils, there is no free lunch, but our little motors in my opinion don’t stress oil much anyway.
Not like a high RPM high performance turbo motor as an example.

[gonesail]

if yanmar specifies 30W then why not use it? I have used Rotella but like the Castrol product too.

[roverhi]

If the weather stayed in the 70s in the PNW, 30 weight would be fine but it occasionally gets into high 80s and more often drops below 60. A multi viscosity oil will cover the temp extremes better. Most importantly it could be the difference in getting your engine started in colder temps.

[Compass790]

I'm with A64 pilot, don't sweat it, both will be fine. I personally use 15w-40 as its meant to be better for short runs & cold starts supposedly. It's also a lot easier to find here.
I stopped obsessing about oil after reading this.Consumer Reports:Truth Motor Oils- July 1996.

[Wotname]

Originally Posted by Compass790

I'm with A64 pilot, don't sweat it, both will be fine. I personally use 15w-40 as its meant to be better for short runs & cold starts supposedly. It's also a lot easier to find here.
I stopped obsessing about oil after reading this.Consumer Reports:Truth Motor Oils- July 1996.

I agree, don't sweat it. Regular changes are way more important IMO.

I use a straight 30 weight in a 2GM20 and ambient temps here range from 50F to 80F. Never have starting issues but I do have good batteries (and compression).

I am a bit pedantic as I change oil about every six months which is possibly overkill as this often means only 20 or so hours between changes.