The Danger of Increasing Viscosity

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In hot climates, nothing. Remember, there's a bigger gap between a 1040 and a 1030 so beware of shearing. Also, since there will be more dense, might appear to be noisier.
The only danger is if you live in a cold climate and the ambient temperature drops below the pumpability of the oil. If it's so stiff that it can't be pumped to the top of the engine quickly, or congeals in the pickup tube and causes a blockage, this is where the damage occurs. Modern oils are so resistant to thickening in the cold, it's not much of a concern above 0 degrees F. I just suggest for safety's sake that if you're in a cold climate, you stick to lighter oils.
So my GM manual for a 2005 3.5L V6, says to use only 5W30 at all temps. Then is goes on to say to NEVER use 10W40 or 20W50. I am tempted to throw in 20W50 with a hefty dose of lucas or STP just to see what happens. My guess is that GM wants to make their recommendation "idiot proof". As a result, they exclude acceptible oils to avoid rare, unacceptible use.
Doesn't the "10W" in both of these mean that oil flows as if it were a 10 weight in cold temperatures? The 30 and 40 come in at higher temps. Someone enlighten me.
10 referrs to pumpability when cold, 40 when hot. 5 is easier to pump cold than 10. If your manual says NEVER, then I wouldn't use it if I were you (although I can't imagine what it would hurt.) They're probably just trying to prevent some dummy in Northern Montana from putting heavy oil in the car in January. Mine 'recommends' 5W-30, but 10W-30 and 10W-40 are also acceptable depending on ambient temps.
CBDFrontier06, The only exception made in the manual is that 5W30 synthetic or 0W30 (no mention of synthetic for this grade) can be used if ambient temperatures are sustained below -20 F. The manual actually differentiates mineral 5W30 from synthetic 5W30 and only recommends the synthetic for cold enviroments. Vehicle is a 2005, Pontiac, MontanaSV6, LX9 3500, AWD.
Well, I have to disagree with them on the part about synthetics. I use it due to the heat in Texas...more resistant to thermal breakdown. If I were you, I'd run Mobil 1 5W-30 - A full synthetic has excellent cold pumpability and provides excellent high-temp protection. Are you in KY, WV or similar?
Actually, I am currently running GC (0W30). My next oil change will be BC (5W40). I am still pondering wether I will dilute BC down to around 12 cSt@100. I have ordered some 5wt oil for that purpose. I am at the junction of KY, TN, VA, WV, and NC - depending on the day. The lowest temp I've seen here is -17F and the highest is 104F.
What is the reason you want to change?
Two reasons: 1. 10W-40 DINO shears down to a 10w-30 anyway after 3K (or so I've been told). [I dont know] 2. In persuit of the "smoother tighter feeling." Sounds silly I know. [freaknout]
I run 10W30 in my car as recommended by manufacture. Start up on a cold morning with conventional oil I would see 100 psi on the gauge. When I started using Synthetic M1 I now see 75-80 psi. Big difference in pressure at start up.
Awrighty, as I recall, the GoldenRod-mobile is a Buick LeSabre. Here is T-Keith's Excellent LeSabre 3.8L UOA run on a 50-50 mix of Chevron Supreme 10W-30 and Delo 15W-40. The drained viscosity, as listed in the UOA, was 11.5 cSt and the virgin viscosity calculates out to 12.8 cSt. So this blend is probably 1 cSt shy of being a true 40 weight, but the UOA is solid data as to the effects of running a heavier viscosity than the standard ILSAC/API Starburst designated 30 weight range of 10.5-10.8 cSt. And, as always, I recommend Pennzoil 10W-40 with the listed P-data sheet viscosity of 13.6 cSt, for anyone interested in running a more fuel efficient 40 weight.
The oilpan design on my Frontier makes oil viscosity MPG loss nearly irrelevant. The lower part of the pan is only about 4 inches wide, running only about half the length of the lower block, and is quite deep (looks a lot like a miniature bread pan) so the crank counterweights never impacts standing oil. Almost as good as a dry sump system. This may be common, I really don't's the first vehicle I've owned with an oilpan designed like that.
I would not consider sump design as a major part of the viscosity related drag. If your crank is routinely slapping oil in the pan, I think that you are using too much oil. Bearing drag and pump resistance are the cause of MPG loss due to viscosity.
I was thinking of running 10W-40 in my 2002 Cavalier in the summer to try and help it with piston slap when my warranty runs out. My car recommends 5W-30, and knowing that most 10W40's shear down to a 30 weight, I thought the best course of action was to use as thin a 10W-40 as I could find that would definetly shear down to a 30-weight. In looking into this, I found that Pennzoil 10W-40, at 13.6 cst, was probably the best candidate for this. So thank you, Blue99 for confirming that this may be a good course of action!
I always wanted to try a 40 weight in my 2004 Sunfire. (2.2L Ecotec). Does the 02 Cavalier come with the Ecotec? I can't remember what year that was introduced (think 03). I'm told the Ecotec in Europe runs a 5w40.
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