15w40 or 10w30?!

TapouT

Well-Known Member
Joined
Oct 5, 2008
Location
Hickory nc
It's time to change the oil in my Jeep Grand Cherokee. It's a '98 model with the 5.2 (318). The previous owner ran 10w30 in it and so did I for the first couple months I owned it and then I switched to 15w40 just because that's what I've always ran in my Jeep's. The Jeep has 177,xxx miles on it. Would I benefit any going back to a thinner weight oil? The jeep holds great oil pressure with the oil that's in it now but I was thinking since it's getting cold outside I should go to a thinner weight. The engine has never made any noise with either weight oil. Also, someone told me awhile back that going with a thicker weight oil would affect my gas mileage and I didn't really pay enough attention to it to see if it did or not. Thanks for the help!
 
I'd run 10w-30 just because the 15w-40 is probably a diesel spec oil. It has different additives and such to deal with soot and resistance to breakdown from diesel fuel, not sure how it fares against gas.

I don't really think we have enough extreme temperature change to really worry about swapping oil seasonally. I run 10w-30 year round.
 
About 40 years ago I ran into a "oil engineer" working for Esso. I asked him what would be best oil to use in the sbc engine I was building (about 450 hp). His advice was to not change viscosity since the engine broke in with clearances based on oil thickness. I ran a diesel 30 w for years in everything with out a problem. Recently I've switched to 10-30w. I don't think it would hurt to go to 40w on a higher milege motor. Try pouring out 30w or 40w @ 0 degrees, it's like syrup. Imo multiviscosity oils protect better when you start cold and drive off.
 
In all my older high-mileage pushrod 6 and 8 cylinders I run 10w-40.
 
On older engines, people say use a thicker oil because it is less prone to leaking through seals and gaskets. My manual calls for 5w30 and thats what I use to run, but now I run Rotella's T6 5w40. Acts like a 5 weight oil when its cold and 40 weight oil when its hot. I don't see NC getting cold enough to switch to a thinner oil IMHO.
 
Thicker oil in an older motor will also help keep the oil pressure up.
 
Changed the oil in it today and went with 10w30 and its holding a little under 40lbs of oil pressure at warm idle and before it was right at 40 or a hair over. Is that okay?
 
That's shit loads of oil pressure.

I think the motor in the TJ is flirting with 10psi at hot idle.
 
Any oil pressure over 10psi at idle is fine as long as it picks up at rpm. I run what the factory calls for in my car and my wifes suv. I run 20w50 in my yota cause it has 236xxx miles and the valve seals are shot. I also substitute 1/2 qt of lucas oil stabilizer at every oil change.
 
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