Originally Posted By: jeepman3071
Originally Posted By: KCJeep
Originally Posted By: TXCarGeek
1999 Jeep Wrangler 4.0 ~97,500 miles
My parents have owned this one since new; oil changes every 5,000 give or take maybe a thousand miles. I can't recall what my father used when it was younger but ever since I have been keeping tabs on maintenance, my dad and I have been using M1 Syn 10W-30 switching between HM and the regular M1 syn. These past two oil changes however, I decided to change it up. First used was M1 AFE 0W-30; it ran fine on this but the low zinc levels concerned me because from what I understand, the 4.0s need higher zinc levels. This past oil change, I went with M1 HM 5W-30; no issues. Most of those miles are city miles with the occasional two hour trip to the beach/random trip on the highway although the past 10,000 miles were predominantly highway miles put on by me. The Jeep will stay in the family; my parents and I plan on keeping it indefinitely. After reading about how awesome GC is, I have been considering it or M1 0W-40. I know these engines can probably run on olive oil because they're so stout but I like synthetics so I won't go to a dino on any application.
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I would want at least a "thick 30" in the Jeep. QS Defy 10w30, Rotella 10w30 would be good, higher ZDDP examples that have proven themselves in the 4.0. I really like PYB in the 4.0 as well, the sky high moly seems to make up for the average ZDDP and they run really quiet on it. The GC should be a good choice as well. The Mobil 1 0w40 was tried by another Jeeper here and he got a sky high iron UOA just like most of the Mobil 1 oils in the 4.0. Since there are so many other good choices I'd just avoid M1 in the Jeep 4.0. While it is not high in ZDDP (and I'm not convinced a broken in 4.0 needs higher ZDDP anyway) Valvoline has also consistently returned very good looking UOA's in the Jeep 4.0.
+1 what KCJeep said.
Living in Texas, I highly doubt you will see any benefit running a 0wXX oil in your 4.0L. I ran a 5w30 in my 4.0L and actually saw some shearing in my UOA during the winter. That is probably the reason a 10w30 is recommended for them from the factory.
If you lived in Canada, a 0w30 might be worthwhile during subzero winter months, otherwise you'd do better with a 10w30 or 10w40. I highly recommend Pennzoil Yellow Bottle 10w30 or 10w40 in the 4.0L. It is the quietest oil I have run in both my 4.0L and my buddy's 4.0L Cherokee. If you want a synthetic, I recommend Rotella 5w40.
Contrary to popular belief, the 4.0L DOES NOT need high ZDDP oils. That theory is only partly true. Some flat tappet cams do need high ZDDP, but only those with high valve spring pressures. The 4.0 has very weak valve springs, and besides break-in on a new engine, or a stroker with an aggressive cam, the motor will run just fine on regular passenger car oil.
I'd also avoid Mobil 1 in the 4.0L. The 4.0L tends to have higher iron wear numbers than most engines. The belief is that a majority of this iron comes from the timing chain, but that has yet to be determined. Some of the 4.0 UOAs with the highest iron wear I've seen have been with Mobil 1. I used Mobil 1 10w30 high mileage in my own Jeep and it consumed a full quart in 3,000 miles. It never consumed oil on other brands and still doesn't to this day. Its just not worth the higher price in my opinion. Stick with Pennzoil Yellow Bottle or Rotella and that engine will outlast the body.
I'm a bit confused. How would a 5W-30 or 0W-30 oil shear but a 10W-30 oil not? Are they not all the same viscosity at operating temperature? From reading the Oil 101 article on the home page of the BITOG, I was drawn to the conclusion that running an "0W-xx" oil regardless of the climate did not matter.