I know this is a deviation, but I have read all these posts and have a question even though I feel like I am quite knowledgeable after all of your help from the past few hours of reading. For my tow vehicle-an Avalcanche 2500 w/ the 496 C.I. motor that never goes north of Tampa. I have never seen colder than 25 degrees. For 6 mo's low is 80 and high is 95. Book says: use 5W-30, but 10W-30 is acceptable from 0* F and up if 5W-30 is not avail. The book says do not use SAE 10W-40, SAE 20W-50 or any other viscosity grade oil not recommended. What do you all think for me in the summer and winter. Motor is stock except K & N filter and only tows a light weight Talon Cat-boat motor and trailer& fuel only 3000lbs. I have always run M1 5w-30 after break in can I go 20W-50? P.S.. this is the motor that sounds like a diesel for the first minute or two after start up. Also any new scoop on this motor noise. You all are the GREATEST!!!!! THnx

Are you are still running the stock 330HP oil cooler? If you are replace it right away. That thing is so small it does hardly anthing. I replaced mine with a combo 3" EMI oil cooler. At minimum find a cooler from a magnum 454 or 502.

This one bypasses until 215*. I have one.

http://www.marine-performance-parts.com/

22-480 Canton Oil Temp Thermostat

JB, personally if this was my vehicle I wouldn't run anything lighter than a 10w40. They (GM) says it's ok to run a 10w30 but not a 10W40. Yhant does not make sense if engine protection is your concern. Both of these viscosities have to meet the same specs at 0 degrees,(remember the W means winter, not weight). The only difference is that a 10w40 will offer more film strength at 210 than a 10w30. GM can't be concerned about cold weather pumpability, (they are both the same at 0 degrees), so they must be concerned about fuel milage.
A 10w40 showing a little more HP loss than a 10w30. (if you can measure it.)
I used to own a Mountaineer, the Mercury dealer said "do not run anything heavier than a 5w-30 or you will bolw the oil filter up. I ran 5w40 in the winter and 20w50 in the summer. Sold it with 110,000 miles on it and never blew up any oil filters. The heavier oil may also help with the start up noise as it will not drain off parts as fast and a heavier oil helps damp out the noise. Ken

thnx minxguy. does anyone think I can go w/ M1 20w-50 V twin in the truck I listed above so I can get the extra zinc?

I have a 68 Chevelle with a 385HP Fast Burn 350CI. The car sits
a lot and on startup with M1 10w-30 I could hear some piston slap.
I switched to M1 V-twin 20w-50 and the slap went away. Also
the engine runs cooler.

15W-50 M-1 would probably work too.

Synthetic oils pump a lot easier and you can safely go up a viscosity grade or 2.

I know guys with Vetters and Vipers using 15W-50.

Jamie, what will help you more than the extra zinc is the heavier viscosity. Your first line of defense against engine wear is oil viscosity. Heavier oils leave a film which is harder to displace. It's when this oil film gets compromised (metal to metal) that reserve boundry additives (zinc and phosphus) come into play. Zinc is also an anti-oxident so lots of it contrubute to the long drain intervales of these types of lubes. Ken

myturn, when you say the engine runs cooler,I am assuming oil temp and not water temp. If your oil is cooler, the 20w50 is not removing as much heat as the 10-30 you were running. Chances are your, piston crowns, exhaust valves, and cyl heads are all running slightly hotter. Remember a properly tuned engine runs at a given temp. Changing the oil does not change timing or jetting, so your combustion temps are the same regardless of the oil viscosity/type that you use. If your oil temps drop, the oil isn't absorbing the heat as well and bringing the heat to the pan. Hence, lower oil temps. Ken

Water temp, though this is subjective. I need to replace the radiator and electric fans. The engine tends to overheat when running in traffic ans A/C on. Turning off the A/C and the engine runs at normal temp. The 20w-50 V twin seems to help this problem; hense, the subjective comment that the engine runs cooler.