Is it safe to go from 10w30 to 10w40???

[DanH1081]

My 1999 3100 Code M has start up piston slap really bad at startup. When I replaced the intake gaskets last december I put some 10w40 in to flush out the engine before I put the good stuff in. With the 10w40 in the piston slap was gone!. I was wondering if it would be safe to use 10w40 in my car. I know the manual says 10w30 or 5w30, however the manual seems to leave out what to use with piston slap I planned on Amsoil or Mobil 1 10w40. My car has 54K miles now. Thanks

[geoffinbc]

Piston slap when cold if fine and normal. Use regular 10W30.

[DanH1081]

I realize its normal. However I want to get rid of most of the sound. I found that a thicker oil such as 10w40 took care of it. I just need to know if its safe to use 10w40 in these engines? Thanks

[Manitcor]

if your noticing piston slap at start up im unsure how the difference between 10w30 and 10w40 helps you. The number after the "w" refers to the viscosity of the oil at 212 degrees F while the number before the "w" refers to the viscosity maintained on cold start. So in reality if you wanted to thicken the oil on cold start you would want 20w30 or 20w40. HOWEVER thicker oil will present issues with oil being pumped properly into the engine.

Personally I go with AMSOil Series 3000 5w30 year round.

[DanH1081]

if your noticing piston slap at start up im unsure how the difference between 10w30 and 10w40 helps you. The number after the "w" refers to the viscosity of the oil at 212 degrees F while the number before the "w" refers to the viscosity maintained on cold start. So in reality if you wanted to thicken the oil on cold start you would want 20w30 or 20w40. HOWEVER thicker oil will present issues with oil being pumped properly into the engine.

Personally I go with AMSOil Series 3000 5w30 year round.

Somehow 10w40 is thicker than 10w30. The technical data shows this
For example, Mobil 1 10w30 is 66.57 CST @40C, 10.5 CST @100C, 10w40 is 91.07 CST @ 40C and14.2 CST @ 100C, as you can see there is a little difference in the thickness between the two. When cold the 10w40 is 24.5 CST thicker when cold than 10w30. According to this data anyways. Then again I am no expert.

[geoffinbc]

You are right. 40 weight is thicker when cold. This is why you proabably dont hear any noise with the thicker oil. But seriously this issue is not a problem and you should still run the recommended oil. Like manticor said the thicker oil on startup will mean that the pump may not flow a sufficient amount when cold. Chances are you will be ok. But I wouldn't risk it for a symptom that is typical of newer engines, in all makes and models not just GM.

Once your car heats up the aluminum piston expands into its bore and the slap is gone. This is why the 10W rating of the oil wont make a difference and 5W oil will proabably never show and symptoms of piston slap either because when the engine is hot the piston is much tighter in the bore.

[EspanolaGuitarista]

I know this may be overkill...

but I have seen some pre-oilers for dirt cheap on the internet. I think a pre-oiler would take care of some of that noise on start-up.

[TurboZ24]

Piston slap on all the GM motors are more due too poor tollerances on the bore/pison size as well as probably piston materials. Every 60 degree V6 I've rebuild has no slap, even those with forged pistons.

[geoffinbc]

It is not poor tolerance. It happens by chance when an engine with the smallest permissable piston is combind with a bore of the largest permissable size. The tolerances are not checked by anyone. If the parts fall into the alowed specifications then they are deemed good to go.

If a person were to match parts together then there would be no problem. However mass production does not allow for it.

[TurboZ24]

It's still an issue with the tollerances GM allows. Obviously it's possible with mass Mfg, since every other manfacturer seems too be successful at making engines that don't sound like diesels at startup.

I understand that is really isn't significant, but when someone buys a new car, which then rattles badly at startup (like the 6.0 in my 2500HD), I'm sure that affect's GM's image. The truck is great, except the 5 minutes of piston slap.

[geoffinbc]

Alot of other car makes have the same problem. You should have herd this Acura Integra I test drove with my cousin the other day. It sounded worse than the 3100 Malibu at work. You could hear it quite clearly even inside. However once I took it out for a burn she was purrin like a kitten.

The biggest reason we are starting to see issues like this is because pistons are made with a looser fit and shorter skirt to reduce drag and inprove engine performance. Even coatings are becoming popular on new engines for just this reason. Several new engines are even starting to use oil squirters and forced oil pistons to inprvoe lubrication to this area which has only been splash oiled in the past. This increase in oil and reduction in heat further increases efficency.

[EspanolaGuitarista]

I work on many different types of cars...

In my opinion, GM sucks. Toyota and Honda build much better cars, unfortunately. If I could afford a brand new Acura TL I would buy one. You build what you can afford. I'm sure this is going to piss some of you off, but that is why GM is in the toilet. That and all those fat-a$$, sausage guzzling, strokes waiting to happen, that make up the typical american work-force. This is off-topic a bit, but many countries don't eat like we do and their companies can build better cars, because they don't pay the health care we do.

[1988GTU]

I thought it was because a teflon coating was used on the piston walls. Then over time is rubs off then you get the slap.

I could be wrong.. ??

[geoffinbc]

Re: I work on many different types of cars...

In my opinion, GM sucks. Toyota and Honda build much better cars, unfortunately. If I could afford a brand new Acura TL I would buy one. You build what you can afford. I'm sure this is going to piss some of you off, but that is why GM is in the toilet. That and all those fat-a$$, sausage guzzling, strokes waiting to happen, that make up the typical american work-force. This is off-topic a bit, but many countries don't eat like we do and their companies can build better cars, because they don't pay the health care we do.

LOL now we are off topic because I am going to add this.

Noth Americans (Canada included) eat to get full. Think of this next time you go to a restraunt and order an appatizer before your meal eat all that decide you "not full" and order desert and maybe a coffee. Same thing happens at home. Instead of eating ALL the leftovers how about you wait till lunch tomorrow. That is why the population is getting fat.

I try and follow the Aisian style. Eat to eat. Just eat something when your hungry and stop. Thats it. Is that so hard? Aisians never go out and eat until they are stuffed. Why do you think their society is so lean? Go there and you will whitness this mentality first hand. It is the way to live.

Honestly my friends laugh at me when I go out to eat with them because I often just order an appatier sized meal or dont eat what I order because I stop myself and let them pick all the fries off my plate. I wouldn't claim that I only eat healthy food because I eat at Mc Donalds but I never supersize and often it is just a Cheeseburger meal.

So back on topic.

No there is no coating on the older motors. The new 660's have them though.

[DanH1081]

So should I be ok using 10w40? I figured a better flowing oil like amsoil would work. I just dont want to cause any problems. The thicker oil softens the blow of the pistons hitting the cylinder wall. I am currently using 1.5 qts 15w50 and 3 qts 10w30, whick according to viscosity calculations is less thickness than 10w40.