GMC 5.3L with the startup rattle

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Jun 8, 2002
Elizabethtown, Pa
Well-the Mobil is obviously cleaning out your engine-that's good, You may also need to add some AutoRx. Mobil1 has a very good additive package with Moly. Chevron Supreme (Dino oil) also has moly-thjat might be a consideration. In any event you might want to go to a 40 wt-it might help the piston slap. 10W-30 Mobil1 won't get the job done. Adding a quart or two of 15W-50 to the 10W-30 Mobil might help. Redline is truly loaded with Moly. I believe one of the posters (Bror) has indicated it stopped the slap.

Mobil Delvac 5W-40, Amsoil 10W-40 could also work. Also check Shaeffers-might be your best choice:
The sludge you see on your drain plug wasn't caused by the Mobil was being removed by the M1. I'd suggest a dose of Auto-Rx or Schaeffer's #131 Neutra to slowly and effectively clean your engine, then try different oils to quiet GM's CSK (cold start knock).

Has anyone tried adding Schaeffer's #132 Moly E.P. Oil Treatement for quieting the slap? I'm not a fan of adding stuff to my oil, but this might be good for quieting the piston slap and for improving 5W-20 oil for anyone who's concerned about using 5W-20.


The material on the plug is ferric in nature as it is attaching to the mag on the drain plug. My reasoning is during the cold start knock it is being generated, I speculate it is a cold clearance issue where there will be very little or no boundry layer lubrication even if the oil get's there. hence the requirement for Moly.

Al makes a good point about the oil weight. 5w30 is the recommended grade and 10w30 is an alternative. I have thought about going with a heavier oil but I don't think it will help the situation IF the wear is the type described above. I was really dissapointed when I changed the M1:(

Originally posted by ajax:
chevron does not have moly in it.

You are right-my mistake

You can check this list out for those containing moly:;f=11;DaysPrune=45

Also-I believe I would be concerned with that amount of iron on the plug. Before the warranty is out I would change the oil and collect that material and take it into the dealer. Also it might be a good idea to get an oil analysis-I'm thinking the iron will be through the roof.

I've purchased a '00 silverado with the 5.3L, it also has the rattle at startup that GM is referring to as piston slap. GM will extend the engine warranty to 100k miles if you ***** enough but milage is close to that now. I've done two oil changes on it.

I've run mobil 1 5W30 and was dissatisfied when I compared the mass of material built up on the plug to cheaper mineral oil it was approx 2X the amount. Certainly not bashing M1 I've used it with other vehicles with excellent results. First is this normal? The end of the plug was covered approx. 1/8" deep. My question is about the additive package and what should I be looking for? I would assume that an EP additive would help the situation. Who loads their oil with moly and such. I cruised the usual suspects but it was unclear if it was in their formula and how much was used.

Hope I'm on the right track. I've got no pretension to a certain brand except for the moly (I think) I've run most brands, never was that concerned.

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