Oil question for KRT or any oil guru

[wdhigh]

Any idea why cummins/Nissan state 5w40 is not recommended past 68 degrees?

Not trying to go against their recommendations, just trying to find a good synthetic oil to run. The dealer has been doing it since they build the included ones into their pricing with Mobil Delvac 10w30. Most all the 10w30 I can find are just syn blend with amsoil being the one of the few full synthetics I can find online.

I’m doubting there is much difference in reality, but figured it never hurts to get a professional opinion with such an expensive engine.

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[Chuck the Duck Slayer]

My guess is that the 5w40 would get to thin at the higher temperatures and not provide the adequate protection needed.

[santeeswamper]

Real diesels run on 15w40 . Mobil delvac is good stuff. Why does it have to be synthetic oil your after?

[wdhigh]

Have had good luck with synthetic in other stuff. I try and not be cheap on stuff I rely on.

Saying it’s not worth the trouble?

[santeeswamper]

I don't think u could tell a bit of difference. My beliefs and my beliefs only.... Depending on the mileage of your motor your not suppose to break a engine in on synthetic oils. Honestly I'd run the Mobil and roll on/delcac/havoline/qs/ so on...

[wdhigh]

I’m at 37k, forgot that in the original post.

[Greenwing]

In summer you want to run a heavier oil (10W40). In winter run a lighter viscosity (5W30 or 10W30). I've done this for 10 years on a 2000 Camry and a 97 Jeep Cherokee and both are running like a top with over 250,000 miles on them. I can always find a deal at Advance or Oreilly's with 5qts and a filter for around $25. For diesel I would run 15W40 year round unless you can find a heavier oil for summer.

[CurLee]

Rotella 15w40 is like Frank's Red Hot, I put that s#&t in everything.

[KRT]

I’m no oil guru and I’d imagine the Cummins engineering group is far smarter than myself but I run 5w-40 Synthetic in mine year round. My Ram with the Cummins was no different. Soon as both trucks hit 5k miles, the oil was changed to Rotella T6 5w-40 synthetic.

[Southernduck]

Why go to the lighter oil in those trucks? We operate in hot conditions way more than cold.

[KRT]

Well 5w40 isn’t much lighter and it’s the closest I can get to 10w30 in Rotella full synthetic.

5w40 for diesel and 5w30 for gas is just fine for the South East.

[KRT]

Plus an engine is only going to run a certain temp no matter if it’s 100 degrees or 32 degrees.

[wdhigh]

Plus an engine is only going to run a certain temp no matter if it’s 100 degrees or 32 degrees.

That’s kinda why I’m questioning the temp range they specify. I can find plenty of 5w40 synthetic available. Might be a waste of money however.

Thanks!