Optimal oil for lethally cold weather, Jeep 4.0L, parked outside

Jun 2, 2021
Arizona, USA
Hi all – If you were moving to the Arctic Circle / South Bend, Indiana at the same time that you acquired a 1988 Jeep Cherokee XJ, 4.0L straight six, 4WD, what oil would you use? Chilton's is defaulting to 10W-30. I assume that's what the manual says, but I don't have it.

It's cold there, cold beyond anything I've ever contemplated growing up near Tucson, AZ. The average lows in Dec, Jan, and Feb are: 23, 17, and 19° F. The average highs aren't much higher: 36, 31, and 35° F. The record lows are basically instant death, or at least instant ball death: -18, -22, and -20° F.

The Jeep will have to be parked outside, fully exposed to these ridiculous temps. Can 10W-30 handle this? I thought 5W spotted you 15 degrees or so. Will that be good enough?

I was thinking of AMSOIL's synthetic. (I liked how mater-of-fact they were in this post, and figured I could do worse than these guys: https://blog.amsoil.com/is-motor-oil-made-from-natural-gas-better/)

Does going with synthetic change your weight preference here?

I would trade it in on one of these:


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In South Bend, at worst you'll need 5W30, 10W30 is probably just fine, South Bend is probably a little colder than Central Indiana where I live, but the average Indiana winter day is like 20 at night and 40 during the day (part of the reason or roads are such cracked up s***), it only gets colder than that a handful of days out of the year, and the extreme cold days where it's like negative 10 or below are a handful of days throughout like a decade and that's not really the sustained temperature that's the coolest temp at the dead of the night on the coldest day of the year.
For the record lows in South Bend a synthetic 5w-30 will do fine. I used Amsoil 0w-30 in my Jeep 4.0 when I lived in Montana, with numerous cold starts going as low as -35F. It made a difference once below about 15F.

That said agree that M1 0w-40 is a good choice; you may also want to consider a 5w-40.
Mobil 1 0W30 will serve you well.
While it will- it might make more noise than normal too. I ran M1 0w30 a few winters ago, and I ended up changing the oil a bit earlier than normal because the engine noise in my Ecotec 2.2 (cobalt). It worked fine besides the noise though. I’ve pretty much used any synthetic 5w30 here in MI during the winter with no issues at at. (Or noise!)
My Jeep sees Castrol Edge 10w40 year round. No issues. Another good option is Rotella T6 5w40.

Be careful driving that 1988 in the salt. She’ll rust out on you fast.
Being in Cleveland which is atleast as cold as South Bend, 5W-30 is what I have been using in Winter lately with the warmer months getting 10w-30 in my older vehicles.
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5W-30 synthetic, and sleep fine at night.
I've started gasoline vehicles at - 40C = - 40F with 10W-30 in Timmins, Ontario, they complained but started. No block heater. No oil related engine issues.