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What Motor Oil is Safe for a 4.0L Jeep Engine and Other Flat Tappet Camshafts?

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Hello all. I've been a proud Jeep xj 4.0 owner for 20 years, a 1988 xj, and 1996 xj. The xj's were no doubt the best vehicles I've ever owned.

I recently acquired a 2003 Jeep wj GC with the 4.0, in great condition, soccer mom owned, and the motor was stealthy quiet. The GC has around 120,000 miles on it, and with the 4.0 track record I should get another 200,000. Due for its next oil change, I'll spend the extra for the good stuff, Mobile synthetic, Mobile filter (or so I thought). It instantly developed lifter noise. I remember this years back when I changed the oil using same thing on my 96 xj with around 120,000, and it developed lifter noise, thinking," well, not all 4.0's can be perfect". Coincedence, that I have 2 motors with the same issue using the same oil. So I hit the internet forums to find out that I'm not the only one, many have had this same issue with different oil types, and that the EPA has had the oil companies reduce amounts of zinc, phosphorus in oil manufacturing to save on Catalytic failure. Could this be the answer to pre mature wear and failure of flat tappet motors? I drained and changed to different oil types with higher z/p hoping solve to this, no such luck, I believe the damage is done. Though after a few minutes warm up the noise does fade away a bit, but is still noticeable, and the motor doesn't have the power it use to. I learned to live with it on the 96 xj, and still runs fine years later, but the GC was gonna be my new and improved upgrade, (again, so I thought). I see this topic with in articles and forums has been well discussed over the years to the point of redundant, so many opinions, but no real solutions to what we should be using in our motors.  Has anyone else here experienced this with their 4.0's? Any thoughts, comments, or facts would be greatly appreciated.

I'm debating my next steps before I tackle the triathlon dash removal for HVAC system repair blend doors, heater core, etc, (really Chrysler). :wacko:

Thank you, Kent

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I purchased a stroker engine from Golen several years back for my Jeep. After some research I had decided to go with Mobil 1 High Mileage or Diesel Truck oils as these have the highest zinc/phosphate levels (ZDDP) available without separate additives. The information is available with a reasonable amount of searching on the web. Apparently, due to effects on converters, zinc is being phased out and replaced with boron and other chemicals at the same time as spring pressures are being reduced with roller cam profiles. Older motors (flat tappet) seem to require significantly greater proportions of Zinc (ZDDP, etc) to reduce wear. It also seems that simply adding zinc to the base oil is not as effective as an oil engineered to contain a balanced profile of both slow and fast burn zinc . In fact, the articles I have encountered seem to show that additive zinc with late-model SAE oils is inferior in protection to oils designed with Zinc as part of the package. I have not seen a spec with a higher concentration than Mobil 1 Diesel truck 10-40 or Mobil1 15-50...only Mobill 1 High Mileage 10-40 is a close second...and available at my local AutoZone.

And BTW, I am not advertising for Mobil...I just found that their products were consistently higher in zinc than other available commercial brands according to the information I was able to find.

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There are two things going on here:  1) the change to synthetic oil and 2) the elimination of zinc from current engine oils...


Kent, I doubt that your switch to Mobil 1 has caused lifter failure.  Flat tappet failure results from no zinc additive during the engine break-in period.  The highest PSI load in an engine is the slightly convex base of a "flat" lifter against the camshaft lobe.  In real terms, the load with the valve springs at full valve lift, aimed at this slight contact area, generates enormous force and puts lubricant and additives to the supreme test!


Zinc acts as a buffer here and reduces friction.  Once the camshaft and lifters have established a normal wear-in pattern, zinc is no longer critical although it obviously helps reduce friction and wear anywhere in the engine! 


The lifter noise you hear, from my experience, is the high lubricity/high viscosity impact of the synthetic oil.  Synthetic oil loosens sludge, which can float through the oil system and restrict oil flow through the lifters in the process. 


I have an interesting account to share...In the late '80s, as an automotive journalist, I studied Mobil 1 and its phenomenal ability to protect an engine and extend engine life.  Everything pointed to a hearty endorsement of synthetic Mobil 1 oil.  However, a common complaint among those converting to Mobil 1 on higher mileage engines was oil seal leaks, most often at the timing cover seal, rear main seal or the valve cover gaskets.  Consumers blamed Mobil 1 and other synthetic oils for these sudden leaks.


In a new or freshly rebuilt and broken-in engine, Mobil 1 will actually extend seal lip life and protect seal contact surfaces for extremely long periods.  On the other hand, the switch to synthetic oil on a higher mileage engine will wash away the varnish on worn seals.  (Varnish buildup was common with the older non-synthetic, petroleum base oils, and conventional oil typically destroyed older design seals over time.) 


Note: Higher mileage seals and sludgy gaskets can begin leaking with the switch to synthetic oil.  For this reason, I am cautious about changing to synthetic oil on a high-mileage engine with signs of sludge inside the valve cover.  A current alternative is Chevron's Iso-Syn formulated petroleum oil with low-volatility molecules (like synthetic oils) in its base stock.


So, back to the story I thought you'd appreciate...We bought a 1986 K2500 GMC Suburban 4x4 in the early '90s, a very clean truck in excellent condition at 74,000 miles.  I thought the 350 V-8 would be a prime candidate for Mobil 1 synthetic oil, and I was promoting the product in my Bentley Publishers books on Jeep and other trucks.  (Not paid for promoting Mobil 1, either!  Like Gutthans, I was just committed to Mobil 1 synthetic lubricants.)  I expected nothing but good results.  It's worth noting that this was a carbureted 350 V-8 truck engine, the last model year before TBI.


Immediately, the engine started to use oil!   Prior to the switch, this engine would take a quart of oil at 1200 miles or so when towing a trailer.  In short order, the engine was using a quart every 300 miles—and no leaks!  I knew this had to be sticky piston rings, as the engine had no other signs of wear.  (Valve guide seals, cylinder compression and oil seals were in good shape.)  I knew this had to be the lubricity and viscous nature of synthetic lube, which flushes out sludge from the rings—and in this case caused them to stick in the process!


Of course, this could have meant an engine teardown, possible "rebuilding" or even a crate motor replacement.  Instead, I stuck with the Mobil 1 and rode out the problem.  It took about 2,000 miles for the rings to free up and seal properly; the oil consumption decreased gradually.  In the end, this engine would go 1500-2000 miles before adding a quart of oil under the heaviest towing chores.  Unloaded, the oil consumption was more like 2500-3000 miles per quart, not bad for a GM 350 V-8 of that era with a lot of weight to lug around!


I believe the switch to Mobil 1 in the Grand Cherokee's 4.0L Jeep inline six began washing away sludge in the lifter and pushrod area and elsewhere.  Synthetic oil will loosen up sludge in all of the "oil cooled" areas of an engine—like the rocker arms and top of the cylinder head.  With conventional motor oil at higher mileage, you can track the engine's high heat spots: They're the areas where sludge builds up from the high volatility hydrocarbon molecules burning up and layering sludge or carbon deposits


The most significant longevity factor with quality synthetic oil is its ability to protect metal parts and reduce friction—plus virtually eliminate sludge buildup and "false seal" varnish on the seals.  As for zinc additive, I just rebuilt the engine top end on the magazine's XR650R Honda motorcycle.  I added a Lucas break-in product to the oil.  The supplement boasts high zinc content and other additives, which the new Hot Cams Stage 1 camshaft and Honda rocker arms need for protection during the crucial break-in period.


As a footnote, most of us recommend breaking-in an engine on non-synthetic oil to seat the piston rings.  The high lubricity, viscosity and film protection of a quality synthetic oil can prevent the friction required to seat new rings against the cross hatch of the cylinders...



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