I am goin to change the oil tomorrow morning in my Z. it has about 75k miles on it, babied actually from the original owner before me. I believe he was using 10w30. i usually used 5w30. Since this is a stock LS6 with 75k should i use mobil 1 10w30 going into the hot summer for better protection and better oil pressure? or is 5w30 fine?

 

Follow the manual is usually the best advice you can find. So, 5w30 is just fine. Mileage aside, it depends somewhat on how the previous owner treated the car, also.

That said, I'm in AZ and it's hot here in the summer months. If I'm changing my oil preparing for the summer months, I have no problems with using 10w30 vs 5w30 as the oil is already thin at 10w when it's 100 degrees at 8 a.m.

The advantage of 5w30 in the colder months is that the oil pressure will come up quicker (to bearings/rings) with 5w than a higher viscosity oil. That means less engine wear. You might see a slight increase in oil pressure when it's cold, probably not much when it's hot.

 

both oils are OK, the 30 (second number) is more relevant for the heat and the first is more relevant for the cold.
I like Amsoil and my Z06 had 0w30 Amsoil from the previous owner so I will maintain that for a while, it is a very nice oil which I have used before. My normal oil is Amsoil 5w 30 and I have used it extensively in summer and winter, towing, traveling and cruising and in pretty much every piece of equipment I have.
Use a good synthetic and I highly recommend adding bypass filtration for extending the life of the oil and equipment!
BTW the 0w30 has been used in roundy rounds for long mileage races so you can see that the first number is not as relevant with heat!

 

I would stick with 5w30. I actually use AMSOIL 0w30 in mine. As mentioned above, the first number is a measure of the oils viscosity when cold, the second when hot (212F). Since a 0w30, 5w30 and 10w30 are all 30 grade oils when hot, in theory, they should all have similar viscosities. The belief that a "heavier" oil such as 10w30 should be used in the summer is based mostly on older mineral oils which required significant amount of viscosity improvers to obtain multi-viscosity performance.

To make a 5W-30 oil, one starts with a 5W oil and adds viscosity improvers. These VIs have the property that in cold temperatures their polymers coil up, contributing little to the base oils viscosity measurements. At higher temperatures they uncoil to reduce the base oils loss of viscosity as it heats up. So when the VIs break down (shear down) one is left with properties of the original base oil (the 5W stuff). Thus over time, a low quality 5w30 could over time shear down to a 5w20, or worse. Good synthetics do not need as much VI as conventional oil so in general don’t shear down as fast. Since a 10w30 will need less VI than a 5w30, again in general it will perform better since there is fewer VI to shear down and there is proportionately more basestock in the 10w30 since it isn’t being displaced by VI. That said, there are other higher costs additives that can offset this effect.

In AMSOIL's fluids, their PAO synthetic basestock has very good multi-viscosity properties so they need even less viscosity improvers and as a result are very shear stable.

A benefit to running a 5w30 over a 10w30 is that your oil will have a lower viscosity at startup and warmup thus minimizing wear. But don't confuse the lower first number and believe that a 5w30 (or 0w30) oil is "thin" when cold. At 100F, a typical 30 grade oil is 5-6 times as viscous as it is at 212F.

 

I like Penzoil Platinum 5w30, or the German Castrol 0w40 in my Z06. Mobil 1 will work if I can't find the other two...

 

Another vote for Amsoil

I use 0w/30 in my 99 coupe with 135k miles, run it hard and it still performs flawless.
Best oil you can use followed by their 5w/30. Been using Amsoil since 1989.
"The first in Synthetics"

 

if it meets the GM SPEC you are fine, regardless of brand, everyone seems to like different brands of oil, go with whatever makes you sleep at night, I personally use 5-30 mobil 1 but may try new Quaker State Q syn

 

royal purple 10-30 in my sti and 2002 z06, if your changing it every 3k or less like me the brand or w.e isnt going to matter! not enough time to brake down anyway

 

I'll share a true test with you guys about a little oil testing of late. I am a firm believer in doing as much testing as possible and let the results speak for themselves.
I have a 2000 4 cylinder Alero DD (BTW the Z06 displaced it! ) which I converted when acquired @ 25000 miles to Amsoil 5w-30 with a bypass filter to keep the oil clean. I put an Amsoil full flow 25,000 mile filter and a 65,000 mile Amsoil bypass filter along with the Amsoil 5w-30 which is a 25,000 mile oil. In this Alero I found that since the oil was kept clean by the filters and it did not use much oil I did not have much room to top off the oil other then sampling time for analysis. 32,000 miles later the additive package was done and the oil needed changing, so the 25,000 mile oil lasted 32,000 miles in a situation where I could not replenish the additive package by sufficient top offs! Hell Yeah
I then decided to test again some of the newer competition and went to Q-Power first still using a bypass filter (not the Amsoil filters) and within 2000 miles I had to begin top offs due to loss of fluid (no system leaks). The bypass was changed every 7000 miles and topped off when the level would drop by ~1/2 quart. At 14,000 miles the oil was not looking real good, and I changed the oil at 20,000 miles with filters. I switched to Pensoil Synthetic and the results not being much different, but magically switching back to Amsoil 5w-30 the oil usage drop is similar to at 25,000 miles!
In another post I just recounted how we towed (8000 enclosed trailer) cross country to the Winternationals with my 6.0 L 2500 HD Silverado (74,000 miles now) again Amsoil 5w-30 with bypass filter and in 5900 miles of trip I never had to top of the oil!
My math tells me everytime I test oils Amsoil comes out on top!
I have not personally tested it at this time against the new Castrol Edge, or other 15,000 mile oils (Except for Mobil 1 a long time ago) but why should I believe their oil is going to be better if they dont believe it that way themselves?
Plus to find out if its a 15,000 oil I have to spend more then for Amsoil 5W-30!
It took me the better part of 2 hours to find a tech that told me QPower, Penzoil, and Shell are all marketed by Shell and only Shell diesel oil could they tell me the TBN specs on? Hmmm?

I have been using and testing synthetics since 1989-90 and switched to Amsoil in 2000 and have not found any better or more cost effective!
Mobil 1 had a better product until the lawsuit with Castrol as to what a true synthetic oil was, after the suit was settled they decided to join Castrol and formulate by enhancing the % with group III base stock...maybe good for their bottom line but IMO for value added!
Just saying!

 

If the oils are at their stated viscosity, then both 5W30 and 10W30 would be the same viscosity when the engine is hot.
The 5W30 just has a lower viscosity to flow better at lower temperatures.

 

5w40 amsoil been using it for years in a modded WS6 and my current Z06. Thinking about trying the penzoil platinum next time it is rated excellent.

 

In theory, true. But, keep in mind that a 30 grade oil has a range of viscosity vice a specific number. The range is 9.30 -12.49 cSt at 212F. Oils can be blended to be in different points of this band and still be labeled as 30 grade oils.

Case in point, I find it interested that according to the data on Mobil 1's website the 5w30 has a viscosity of 64.8 cSt at 100F and 11.3 at 212F and the 10w30 has 62 cSt at 100F and 10 cSt at 212F. So, based on that, the 10w30 has a lower viscosity both "hot" and "cold". Go figure. I suspect that at really cold temps, their 5w30 will outflow the 10w30 (assuming those numbers are correct), but not sure what point the crossover occurs.

 

How does the Amsoil compare to these numbers just for comparisons sake?

 

I've been using Amsoil for years. I use 5w-30 in the Z06 and 0w-20 in the Scion tC. Don't put many miles on the Z as the tC is my daily driver. Current oil has almost 10,000 miles on it, level is still where it was the day I changed it and it's not very much darker than it was out of the bottle.

I'm an Amsoil dealer myself and I even tell people that Amsoil is cost effective only if you use it with extended drain intervals. Changing at 3,000 or even 5,000 miles is a total waste of money if you use Amsoil

 

Here's a viscosity table of data taken from AMSOIL's and Mobil 1's websites for GM 4718M labeled oils. Note that I'm also adding High Temp High Shear rating which is a measure of the oil's performance in high temp high shear use as may be seen in the bearings. This rating is a viscosity at 300F and demonstrates how well the oil holds its viscosity as it heats up to extreme conditions.

Viscosity (cSt) 100F, 212F, 300F
M1 0w30 63.1 11.0 2.99
M1 5w30 64.8 11.3 3.09
M1 10w30 62 10 3.14
AMSOIL Sig Series 0w30 56.56 10.3 3.2
AMSOIL SAE 5w30 60.7 10.8 3.2
AMSOIL SAE 10w30 63.3 10.8 3.2

There are several points that I find interesting/illustrative in this data.

Based on Mobil's data, if I wanted the best "cold" (and using 100F as that basis) flow I'd use their 10w30. If I wanted the one that maintained its viscosity best as it heated up, I'd use their 10w30. And, if I wanted the one with the lowest viscosity "hot" for improved fuel mileage, I'd still use their 10w30. All somewhat contrary to their advertising and my gut opinion.

The data provided by AMSOIL correlates to a more traditional view in that when "cold" the viscosity drops from 10w30 to 5w30 to 0w30.

The oil that has the lowest "cold" viscosity and maintains its viscosity best over the full temp range 100-300F is the AMSOIL Sig Series 0w30 (which was my choice for my 06 CTS-V daily driver).

To use Mobil's 0w30, in theory to improve fuel mileage, you sacrfice high temp high shear performance (2.99 vs. 3.09 or 3.14). AMSOIL's blends all retain the same high temp performance (3.2). In my opinion, this is the most illustrative data point of the variances in the basestock. Some may argue that these differences are small and that not many bearings on street cars will see 300F. These are valid points and I believe that this then comes down to a personal choice to pay a little more for the 7% variance in HTHS between the AMSOIL and M1 0w30.