I think 15w-40 in my jeep was a mistake

[Lyman]

I have run 10w30, 10w30 HDEO, and 15w40 HDEO in mine. I could not tell any difference between them. I normally run the Rotella T6 10w30.
I did overfill once and noticed right away. Very sluggish till I corrected it.

 

[JavierH19]

My yukon started perfectly fine with 15W-40 in a cold snap at about 20f. Maybe it took a hair longer almost nothing. My sierra struggled but that was because of the battery which crapped out soon after.

 

[Chris142]

If your cold starts are 90F then yes it's fine. At least during the daytime. I don't know how cold your nights are. You might need to do a cold start at night or early morning sometimes. I wouldn't worry about cold starts that are 70F or higher. Probably somewhat colder would be fine too.

I'm in the Mojave desert. Our temp swings are not that much.. maybe 30 degrees at the most. So if it's 110 in the day it may get down to 80 at night.

When I left the house @7a this morning after putting on my seat belt I turned on the ac as it was already uncomfortably warm to me.

Winter might get down into the freezing temps occasionally but I will have drained this 15w 40 by then and put in something more suitable.

 

[MNgopher]

For what its worth, I ran everything between 5w30 and 5w40 in my 4.0 in my old Cherokee. Never noticed a difference and never had any difference in fuel mileage even running 40 weight oils at -20F (at those temps is always bad regardless of oil weight...). The 4.0 was really indifferent to the various oils I threw at it, including diesel 40 weights...

Can't help but think there is a lot of perception happening, but recognize YMMV...

 

[cheesepuffs2]

I never said it wasn't safe at 90F during the day. In fact I said it is. Stop putting words in my mouth and read more carefully. You're really getting on my nerves now.

I never said that you implied it wasn't safe at 90 degrees during the day. I said you implied that that is when it becomes safe, which is misinformation because 15W is safe to cold start at ambient temperatures far, far below that.

 

[addyguy]

15W-40 is not too thick for a 4.0L I-6 in summer heat. This engine is just a ‘tinkered with’ AMC 258 I-6, that is descended from the 199 I-6 of the early 1960’s.
In 1976 at least, the engine oil chart gave multi-visc and straight-grade recs; including 20W-40, 10W-40, and SAE 40. I’d imagine most 60’s/70’s AMC’s ran 10W-40, as that was considered ‘best’ back then.
The 30-grade section called for 10W-30 and SAE 30; and that ‘tended’ to be cheaper oils, so a lot probably ran that bc AMC’s were an economy car.
So either will work, and the engine should run the same on both.

 

[ripcord]

15W-40 is not too thick for a 4.0L I-6 in summer heat. This engine is just a ‘tinkered with’ AMC 258 I-6, that is descended from the 199 I-6 of the early 1960’s.
In 1976 at least, the engine oil chart gave multi-visc and straight-grade recs; including 20W-40, 10W-40, and SAE 40. I’d imagine most 60’s/70’s AMC’s ran 10W-40, as that was considered ‘best’ back then.
The 30-grade section called for 10W-30 and SAE 30; and that ‘tended’ to be cheaper oils, so a lot probably ran that bc AMC’s were an economy car.
So either will work, and the engine should run the same on both.

Pretty sure Chris has a modern Landcruiser with the v6 4.0.

 

[Carlostrece]

I'm in the Mojave desert. Our temp swings are not that much.. maybe 30 degrees at the most. So if it's 110 in the day it may get down to 80 at night.

When I left the house @7a this morning after putting on my seat belt I turned on the ac as it was already uncomfortably warm to me.

Winter might get down into the freezing temps occasionally but I will have drained this 15w 40 by then and put in something more suitable.

Then your fine.