KnoxJeep

Can I run it in the 4.0?

Manual calls for 5 or 10w 30...

Gatchmo5710

Yes u can.

RTorrez1

Yes you can.

djb383

0W40 or 5W40 would be better than 10W40.

1991Jeep_Man

x2. My '91 XJ 4.0 loved the Rotella 5w40 I put in it. Cold starting was a lot easier. However, with summer returning and hot temperatures likely, I may go to 10w40, then go back to 5w40 when it cools off.

But yes, 5w40 is da bomb.

djb383

0w40, 5W40, 10w40, 15W40 and straight 40 are the same vis at operating temp. There's not a huge difference in operating temp, summer or winter, in a water cooled motor. The lower the 1st number, the quicker the flow at any temp below operating temp.

1991Jeep_Man

I understand this, but with my XJ having so many miles and burning oil, I suspect the easier flowing 5w is getting past worn valve guides. During the winter the colder months, I figured this effect would be minimal due to the colder temps. But with hotter temps, I figure switching back to a 10w40 will help a little considering the lowest temp during summer is only like 70*, and when I first start the Jeep it can be as much as 80-90*. But who knows... I am no expert and my logic may be flawed.

Edit: Just a note here. I am totally agreeing that 0w and 5w are best because they do flow faster when the engine is cold. I will not argue against this because it would ignorant to do so.

djb383

The way I look at it is, oil needs to flow as quick as possible to the valve train, as the valve train is the furthest point from the oil pump....but the valve train needs lubrication asap. If a little oil gets past worn valve stem seals, u just get a little extra momentary upper cylinder lube and maybe a little puff of smoke at start-up. Dry seals wear more/quicker than wet/lubed seals.

mentalbreakdown00

oh boy not again

TheJerm

A 4.0L will be happy as long as there IS oil in the pan. Ive tried 10w30 and 10w40 and seen no difference with a 215K+ motor. So i run 10w30 in the summer, 5w30 in the winter

djb383

I bet it's just gettin' started.

nofords4me

10w40 is the only oil my xj actually likes, the valvetrain is quiet and no leaks. When I use 5w30 I'll lose about 1qt a month on floor.