Amsoil 0W30 and Piston Slap

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Aug 23, 2002
Pacific NW
Engine is a Jeep 4.0L inline 6. Basically a tractor motor. It has 160K miles and had piston slap when I acquired the vehicle at 110K. These motors are notorious for it. Until '99 they were made from very aged castings with monstrous tolerances, and often with mismatched piston/cylinder combinations.

So, into those works I may have cast a $40 wrench. Just switched from M1 10W30 TriSyn to Amsoil series 2K 0W30 (hey, it seemed like the best stuff). Now I'm concerned the light weight isn't providing enough cushion for this old dog's piston slap. Like I said, it's always been pronounced, and is common for this motor, but since the change it has definitely become worse.

The series 2000 question: Do I stick with it, try some kind of cushioning additive, or just switch entirely, draining the $40 Amsoil to my lawnmower oil supply?

I'll have the final M1 analysis in a week or two.

I notice that a lot of people who have piston slap in LS1 f-bodies find that when they switch
from a 5w30 to a 10w30 the noise gets lessened. They also see a further reduction in the
noise if they go from a 10w30 oil without moly to a 10w30 with moly. That moly in the oil
helps to cushion the piston slap I think.
Thanks Patman. I'll toss out some history just in case.

Long ago in a warmer climate I used straight 30W Castrol HD dino in this engine and of course piston slap was much less. Moving to Castrol 10W30 dino increased consumption and slap, but I was used to it to some degree and had moved to cooler climes.

Switching to synthetic, I continued with 10W30 (Mobil1 trisyn) for three changes (1000/6000/3500mi intervals). Piston slap did seem to increase slightly from the dino. Consumption jumped to 20oz for the first 1000 miles then stabilized at ~4oz/1000miles thereafter. Analyses didn't show any excess wear though I'm unsure what would show up.

Now, immediately after the switch to 0W30, I'm concerned piston slap has reached a harmful level. I only have my ears to go by but it's certainly louder and sustained. I can't afford a teardown right now to confirm suspicions. What I'm hoping for is some guidance in selecting a series 2000 compatible additive (Moly or other) to cushion the slap, or perhaps a pointer to a more appropriate oil. I hear and have read here that Redline might be a solution. Any firsthand experience reducing slap with high-quality synthetics would be appreciated. This is a tractor motor for gosh sake, but a well-cared for tractor motor.

(Realizing too late he based oil choice for this vehicle on incomplete/forgotten criteria.)
(Or... It sounded sweet for use in a new 32valve 8Krpm engine. Too bad I don't have one.)
If you don't want to drain out the oil right now, how about this? Change the oil filter and top
up the oil with some Amsoil 20w50. This should thicken things up a bit until your next oil
change. I hate to see good synthetic oil get drained out too early!
I agree with patman. But I would say to buy a piece of plastic tubing (the hard stuff) which will fit down the dipstick tube. It'll take about an hour to siphon off a quart. You could use the Mobil 1 15W-50. Also Shaeffers makes a Moly additive.
Or with the engine cold, take off the drain plug and let about a quart come out. With the engine cold you won't burn yourself trying to put the drain plug back on (which is always tricky to do when the oil is coming out so fast)

If the piston slap occurs when the engine is cold, you need to run a thicker oil. I'd recommend mixing in two quarts of the amsoil Series 2000, 20w-50. You could also use mobil 1, 15w-50, although the additive chemistry is different.

Do not try using any additives with this oil - you have no idea what sort of reaction you will get. For example, one local guy dumped in a pint of marvel mystery oil into amsoil and the mixture gelled up. NOT a good idea ...

David, I also have a 4.0L Cherokee, with 82K miles on it. I hear a little piston slap upon a cold start-up, but it goes away after a couple of minutes. I am currently using 10W-30 Castrol GTX.

From what I've heard, these engines will run over 200-300K miles while making this noise. It dosen't seem to be anything much to worry about....
I guess I'll be the dissenting voice and say that the effects of a thicker oil will only lessen the noise to a certain extent.

The problem is that the piston is worn and in cold weather it's shrunken as well. The gap and resulting play is what makes the noise as the piston rattles. I suppose a thicker oil may provide some cushion on the cylinder wall ... but I can't imagine it is too effective.

The piston slap in my Honda all but disappeared when I switched to Red Line oil ... and I was using their 5W30 in the winter.

So, unless you think there is something magical about Red Line's polyol base oil, I think you have to credit the moly barrier additives. My theory is that the moly plates up on the worn piston skirt and actually fills that gap which prevents the rattling when the piston is cold.
A thicker oil simply cannot do that.

So, I'd recommend using any weight you want ... as long as the oil uses a moly anti-wear package. Amsoil with its heavy ZDDP package may be good at PREVENTING piston slap, but once it's occurred, I don't think it will be terribly helpful.

--- Bror Jace

Originally posted by OneQuartLow:

The series 2000 question: Do I stick with it, try some kind of cushioning additive, or just switch entirely, draining the $40 Amsoil to my lawnmower oil supply?

I'll have the final M1 analysis in a week or two.


I would stick with the oil for a decent amount of time. If you ran the mobil 1, and used analysis do the same with the Amsoil so you can compare.

This would make the most sense. I have a 97 F150 with some piston slap. It does it with regardless of oil choice. I have been running 0w30 for the last 3 years. It seems no better or worse running this oil. My analysis numbers are looking good considering I haven't changed my oil in over 1 year.

Also I don't understand how the oil would effect piston slap, since it is a function of the pistion, which gets no oil? Now if it were noisy lifters or gears I would consider heavier oil, but not in this case.

Actually since the oil circulates faster, you piston slap should decrease at a faster rate in the cold than with a heavier oil that take longer to warm up.

I say give it a try through the winter, do your analysis on it and let us know the results.

goood luck,

msparks, these motors can slap all the time. (those with the problem anyway) It's most noticable at idle where people honestly believe it's a diesel. It isn't just a startup or cold day thing.

The reasoning behind using a higher viscosity oil to reduce slap is that the thicker oil will better cushion the piston skirt's impact on the cylinder wall. That's even worked previously with this motor. There were audible differences when I used different weights of Castrol.

However, (insert funeral dirge) in this case the heavier oil made little difference. Yesterday I drained 2qts & then topped up with series 2000 20W50. It may have quieted slightly but after 20 minutes of driving the slap is still pronounced.

My next step is to isolate the offending cylinders. I'll probably get an early analysis to check blow-by, maybe even try some moly additive, but the series 2000 by itself doesn't seem compatible with old red.

[ August 28, 2002, 11:50 AM: Message edited by: OneQuartLow ]

Originally posted by OneQuartLow:
msparks, these motors can slap all the time. (those with the problem anyway) It's most noticable at idle where people honestly believe it's a diesel. It isn't just a startup or cold day thing.

I see, mine just does it when it's cold. And goes away after about 1-2 minutes when the pistons warm up. Of coarse they have replaced motors in the past, but I am well out of warrenty. I don't think that my pison slap will hurt anything, just noisy when I first start up.

Good luck

You might want to try the Schaeffer's 15W40. With the "built-in" clearance you already have, this weight might work well and it already has Moly. It's a blend with about 17% PAO for cold weather and extended temp ranges.
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