# 05 GTO Oil: 5 or 10?



## Starrbuck (Dec 13, 2005)

I just did my first oil change at ~1k miles in my 2005 GTO. The manual recommends 5W30 Mobil 1. However, the dealer I purchased it at recommends 10W30 Mobil 1 all year round here in the warmer climate of Texas. The service guy even owns his own 2005 GTO and runs 10W30 in his. They acted like I was nuts when I asked for 5W30. Do you think it is okay to run 10W30 or should I stick to 5W30 like the manual states?

Thoughts?


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## PrivateG (Nov 25, 2005)

Well I used 5W in mine for now but come summer time anyway i will go with synthetic anyways. And I live in Houston.


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## Starrbuck (Dec 13, 2005)

Mobil 1 is synthetic. I'll always run Mobil 1. The question is 5W30 or 10W30.


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## Toro Toro (Oct 30, 2005)

I would tend to go with the 10w30 in the hot Texas summer months but probably stay with the mfg. for the rest of the year. The type of driving your doing will also need to be taken into consideration.


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## Subdriver (Dec 28, 2004)

I'd ask them why they recommend a 10w30 vice a 5w30 and see how much they know. :lol: 

The first number in the rating, e.g. 5w or 10w is the oils weight when "cold". This viscosity is normally measured at 40C, or 104F, so it isn't really "cold". The second number, e.g. 30, is the oils weight when "hot". This viscosity is normally measured at 100C, or 212F, very near the normal operating temp of the oil. 

Both 5w30 and 10w30 are 30 weight oils when warmed up, so essentially they are the same. Many people incorrectly read the weight classification when cold and think the oil is less viscous due to the lower number. But, the rating system is different at 40C than at 100C. A 5w viscosity at 40C is about six times higher than a 30 weight at 100C. For example, Mobil 1 5w30 viscosity at 40C is 64.8 cSt, and at 100C is 11.3 cSt. Switching to a 10w30 will result in an even thicker oil at startup, robbing HP, fuel economy, etc, and will provide little to no benefit when hot.

Viscosity Improvers are viscous chemical compounds called polymers or polymeric compounds that decrease the rate at which oils change viscosity with temperature. These viscosity modifiers extend a motor oil’s operating temperature range and make multigrade or all-season oils possible. The VI is measured by comparing the viscosity of the oil at 40°C (104°F) with its viscosity at 100°C (212°F). VI can provide insight into an oil’s ability to perform at high and low temperatures. Petroleum-based motor oils require the use of viscosity improvers to meet the low temperature requirements of SAE 0W, 5W or 10W and the high temperature requirements of SAE 30 or heavier oil. Synthetic-based motor oils have a naturally high viscosity index and require less viscosity improver additive than petroleum oils.

Given this, for a good synthetic, there is little benefit to running a 10w30. But... it probably won't hurt either. :cheers


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## Iceddecaf (Dec 14, 2004)

Say Subdriver, just wanted to say thanks for all your posts on oils and oil filters. I've saved them and stored them in the GTO file. Great information! I now have confidence in knowing what oil and oil filters to use. By the way, I'm a fellow Washingtonian. Keep up the great posts!


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## b_a_betterperson (Feb 16, 2005)

Starrbuck, just for grins, go back to your dealer with a printout of Subdriver's reply -- then tell them to change it with 5W-30.


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## Starrbuck (Dec 13, 2005)

Yes, Subdriver, I've seen some of your other posts and the info is appreciated!

I think you've made me feel better when you said:



Subdriver said:


> Given this, for a good synthetic, there is little benefit to running a 10w30. But... it probably won't hurt either. :cheers


I think it's safe to stick with this 10W30 until the next change, but I'm going to go elsewhere to get 5W30 or buy it on my own and take it with me to the oil change place.


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## Subdriver (Dec 28, 2004)

Iceddecaf said:


> Say Subdriver, just wanted to say thanks for all your posts on oils and oil filters. I've saved them and stored them in the GTO file. Great information! I now have confidence in knowing what oil and oil filters to use. By the way, I'm a fellow Washingtonian. Keep up the great posts!


Thanks! Glad to know I've helped. :cheers


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## Subdriver (Dec 28, 2004)

Starrbuck said:


> I think it's safe to stick with this 10W30 until the next change, but I'm going to go elsewhere to get 5W30 or buy it on my own and take it with me to the oil change place.


I agree. I certainly wouldn't drain it and waste the time and money to replace it. Just realize it will be a little thicker when cold so be gentle with it until the oil is warmed up.


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