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Discussion Starter #1
I just know I'm going to regret this, but I am curious about something.

I have owned 5 cars with GMs 3800 engine, which has always been a fairly bulletproof engine. I have always used either 10-30 or 10-40 oil in them, and that is what the manual recommended.

On one of my current cars, a late model Grand Prix GT, I had been lazy and was letting Walmart do the oil changes. This time I planned on doing it myself but the store was out of 10-30 Castrol, so I picked up 10-40.

After doing the oil change, and looking in the owner's manual to see how to reset the "change oil" light, I read that they recommend 10W-30, with 5W-30 acceptable in colder temperatures.

Here's the good part. The manual says that under no circumstances should 10-40 or 20-50 be used. I understand the idea behind recommendations, but I have never seen a manual so adamant about NOT using certain types. It has me quite curious since 10-40 was an acceptable oil for many years on this engine.

Any ideas why? Is this just an emissions or gas mileage concern? Or could there be a legitimate concern with higher viscosity? Did they change the specs of the oil pump? Are the seals different?

Don't just share opinions of what you think is the right thing to do. I would like explanations of your logic and reasoning.
 

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Larry -

Opinion: I'd use the recommended weight/viscosity of oil.

Reasoning: I have lived long enough to remember when 50,000 miles was usually the time to consider an overhaul. The reason today's engines can go four times further (or longer) before that overhaul is because manufacturing techniques and tolerances have improved. Those same improvements have drastically changed the oil requirements, too. Straight viscosity has been replaced by multi-viscosity. Non-detergent has been replaced by high detergent. Engines now come from the factory with fully synthetic oil, no "break-in" required - unthinkable thirty years ago.

Conclusion: I can't tell you the exact reason for the change, but I feel confident that there is a reason, and a darned good one at that.

YMMV.... ;)
 

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If you want to get into some serious discussion of the whys and wherefors of oil, check out Bob is the Oil Guy.
 

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10W-40 isn't going to hurt, they say it will reduce milage a bit. The engine will be fine, however if you are having any kind of warranty work done and the dealer is a **** Then you could have problems. Its up to you but that would be the only thing I know of.
I have the same in the Owners manuel of mine. Wife and I bought in 06 and they both say 5W-30 only. One is the same engine you are talking about and the other is a V8.
 

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Thin Oil

Better halfs Mountaineer calls for 5-20. Scares me to death but 150,000 miles later, I dunno. Maybe Mr. Tim can come in and enlighten us. Jim
 

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Willie G nailed it. Tighter manufacturing tolerances and thinner motor oils with higher film strength can flow better in tighter spaces, yet provide plenty of protection. 90% of wear occurs on start up, and the thinner oil circulates faster getting in all the tight spaces and provides better cooling.

10W40 is also going to contain more polymeric viscosity improvers, less lubricating oil and more likely to shear than 10W30 oils.

Always follow the vehicle manufacturer oil specifications for SAE, API, ASO and whatever else they list, oil filters, and recommended oil change intervals. And if you use oil and filters that greatly exceed the minimum specifications, then you can extend the intervals up to the maximum oil and oil filter company warranted change intervals.
 

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two easy and basic reasons

# 1---10-30 saves fuel and that figures into GM's fleet mileage program with the FED GOV.....

#2--- the oils with widers spread in viscosity have been found to produce more oil deposits in the engines (aka sludge)


Ford has gone to 5-20 oils in a effort to further reduce power needed to pump the oil , haveing said that the clearances inside the motors also need to be reduced to maintain enough pressure , so dont try to run oil thats not designed for that motor. A ford motor with 10-40 in it will tick like a time bomb because the lifters are not clearanced for heavy oil!
 

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Discussion Starter #9
Wow, some really well thought out reasons. I know that engines are built to much tighter tolerances than they were decades ago, but I hadn't figured that this motor was built much differently than something like my 95 was, since they were both 3800s, but then again, why not.

And I hadn't figured that clearances would demand a thinner oil, especially with the valve train.

I certainly had planned on following the manual on the next oil change. I was just trying to determine if I should leave this oil in, or waste it and put in the proper viscosity.

I will have to think these responses over for a bit.
 

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Larry at temps above 50*f I would not think twice about 10-30 or 10-40 as long as your not running it for 5~8000 miles. Like I said it forms deposits only on long drain intervals or short trips where the motor/oil never gets hot. In my duramax deisel truck I run a combo oil for the winter 1 gallon of rotela 5-40 syn and 6 qts 15-40 dyno rotela , I get faster oil pressure and lower peak oil pressure then strait 15-40. Very rare is it below 0*f here in Mass so I realy dont need the thin oil .
 

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Wow, some really well thought out reasons. I know that engines are built to much tighter tolerances than they were decades ago, but I hadn't figured that this motor was built much differently than something like my 95 was, since they were both 3800s, but then again, why not.

And I hadn't figured that clearances would demand a thinner oil, especially with the valve train.

I certainly had planned on following the manual on the next oil change. I was just trying to determine if I should leave this oil in, or waste it and put in the proper viscosity.

I will have to think these responses over for a bit.
If you are under warranty, I'd change it out quickly in case you ever have a claim under warranty. If not, then I'd change it out around 3,000 miles.

I couldn't find any used oil analysis for Castrol 10w40 at Bobistheoilguy.com, but the Amsoil motorcycle white paper http://www.amsoil.com/products/streetbikes/WhitePaper.aspx?zo=1181889 shows that the supposedly more shear resistant Castrol synthetic 10w40 motorcycle oil has the second poorest HT/HS value and shears to a 30 weight oil. So the 10W40 Castrol conventional passenger car oil may also shear to a 30 weight in your car.
 

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Discussion Starter #12
No, I'm out of warranty. I'm thinking along the same lines, to let it stay in there for a few thousand and suck up some dirt, and then change it. I normally run it until the light tells me it's time.

I don't drive the GP much in the summer, only when it is raining. The Mustang GT and the Wing get most of the attention. So this fall I will change it for the winter. It should have a couple thousand on it by then.
 

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My wifes Honda Hybrid recommends 5-30W synthetic and to change every 10K

I do

Next question
 

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Only run 5w-30 in my GM cars with that series of V-6. That little V-6 is great! I have about 140k miles on my 96 and I think it's the best engine in an American made car, even if it was made in Canada.

I have heard stories of people running 10w-40 in the 3.0-3.8L GM V-6 and needed a new engine before 100k miles. That is just stories or rumors. All I know is that if the manufacturer says 5w-30 then that's what I use. As for using 10w-40 for one oil change I can't believe that it would hurt. I've done it myself a couple of times because I'm getting old and forgot when I bought the oil.
 

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I just know I'm going to regret this, but I am curious about something.

I have owned 5 cars with GMs 3800 engine, which has always been a fairly bulletproof engine. I have always used either 10-30 or 10-40 oil in them, and that is what the manual recommended.

On one of my current cars, a late model Grand Prix GT, I had been lazy and was letting Walmart do the oil changes. This time I planned on doing it myself but the store was out of 10-30 Castrol, so I picked up 10-40.

After doing the oil change, and looking in the owner's manual to see how to reset the "change oil" light, I read that they recommend 5-30, with 10-30 acceptable in colder temperatures.

Here's the good part. The manual says that under no circumstances should 10-40 or 20-50 be used. I understand the idea behind recommendations, but I have never seen a manual so adamant about NOT using certain types. It has me quite curious since 10-40 was an acceptable oil for many years on this engine.

Any ideas why? Is this just an emissions or gas mileage concern? Or could there be a legitimate concern with higher viscosity? Did they change the specs of the oil pump? Are the seals different?

Don't just share opinions of what you think is the right thing to do. I would like explanations of your logic and reasoning.
Larry: Besides good pumpability at lower temperatures, tighter engine clearances, etc., here is another reason for the recommendation of 5W-30 oil in your Grand Prix GT. Does it have GM's Active Fuel Management? I have a late model Grand Prix GXP with the 5.3L V-8 and Active Fuel Management. The owners manual like yours specifically states to use no other viscosities except 5W-30, and that statement is printed in black bold type. An exception is temps of -20 F when a 0W-30 or 5W-30 synthetic is recommenced. 10W-30, 10W-40, and 20W-50 wt oils, dirty oil, or the depletion of the anti foaming agents in the oil will significantly impair the operation of the active fuel management system. The AFM system uses engine oil to shift spring loaded pins in special roller lifters to activate or deactivate the system. The oil circuit (Valve Lifter Oil Manifold Assembly) used for active fuel management system even has its own small oil screen to filter out any contaminants in the system.
 

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Discussion Starter #17
Murf,
No, mine doesn't have the AFM. I don't have the same engine you have. Your manual specifies even lower viscosity oil than mine.

I guess this is a lesson learned. For decades, 10W-30 and 10W-40 was a safe bet for any car, and I got lazy and didn't look it up. From what I am reading here, it seems that nearly every car made today specifies only lower weight oil.

I did the Mustang GT yesterday. It calls for 5W-20. I had actually looked that one up in the manual because we just bought that car. But I still didn't read far enough. What a shocker. That sucker has a 6 1/2 quart sump!!!!!:shock: Now I have to go back to the store and buy more oil. One thing's for sure. That car sure isn't going to get synthetic!! With the price of some of those synths, It would cost almost $70 just for the oil and filter. I would have to take out a loan just to change the oil. ;):lol::lol:
 

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Larry, just came acroos this thread...
The above posts are all correct.
I am a dealership auto tech.... and can tell you that oil viscosity has become a big issue in the past several years.
Manufacutrers are constantly tightening the tolerances in the engines to meet emissions, and the engine will go by the same name, however if you were to do internal engine work, you would need the specific engine info. because internal components will be different.
Also, with the introduction of fuel management, variable valve timing, etc., now most vehicles are sophisticated enough, to calculate viscosity, by temperature, flow rate, etc.
Any car with the above will actually set trouble codes for incorrect oil, because it affects variable valve timing solenoid and cam phaser operation, which affects emissions.
In your particular case it is simply a matter of components that have been tightened up, therefore a thinner viscosity oil is needed.
The recommended viscosity and grade (the SF, SG, SJ designations) should be stamped on the oil caps of each vehicle.
On new or late model cars if it says 5-20 or 0-20, that's what I would stick with, or you will get a malfunction indicator light ( as it is now called), and a trip to the dealer will be needed to turn it off, even if you drain and fill the oil to the correct one. Eventually the light will turn off, but that will require several good trips as they are called, warm up counters etc., all part of OBDII.
Don't ya just love the government?:bow:
Ya know if you think about it, the government now dictates what oil to use in our cars..... How American......:eek:4:
 

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Discussion Starter #19
Thanks for the great info adventurer.

I'm not sure I would go so far as to agree that the government is dictating our oil usage. The EPA only sets the emissions and fuel economy standards that have to be met. It is left to the manufacturers to devise the engineering methods needed to meet those requirements.

I realize that these strict standards have made our cars incredibly complex, but their is a bright side to it. Engine development had been moving along at a snails pace for a long time. If it weren't for the EPA, we would probably still be cleaning carburetors and replacing points and distributor caps. It's hard to argue against the good that has come out of those requirements. And that doesn't even take into consideration the incredible power increases that have come as a result of all this R&D.

At least this is one time that change doesn't cost any more. I can live with that.
 

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pretty simple Larry tighter tolerances on these newer engines needed more positive lubrication epically on start
 
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