I am tired of everybody saying 20 wt oils exist only for gas mileage

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quote:

Originally posted by Mamala Bay:
It's hard to compare thin & thick with quality synthetic oil because they are thinner to begin with than dino.

Actually I don't think this is true in the strict sense of the word thinner. A 10.5 cSt synthetic is just as thick as a 10.5 cSt dino. The synthetic does flow better and in a sense is thinner in that it has a much narrower molecule size distribution, or so I seem to recall.

Rather than risk a thin 5w20, I would go synthetic 10w30.
 
And this is why oil is so difficult. At 100 C two oils may have the same viscosity. The same oils may differ widely at 10 C and below and may differ widely at 150 C and above. This is why we have to compare viscosity only at a certain temperature.

The viscosity can also be defined by the application. My Ferrari runs much cooler with the 20 wt Mobil 1 than the 40 wt Shell Helix Ultra. My operating viscosity is about the same as the "thicker" oil at the higher temperature. My oil is NOT thinner driving to work than with the 40 wt oil. I get about the same pressures with either oil.

aehaas
 
quote:

The viscosity can also be defined by the application. My Ferrari runs much cooler with the 20 wt Mobil 1 than the 40 wt Shell Helix Ultra. My operating viscosity is about the same as the "thicker" oil at the higher temperature. My oil is NOT thinner driving to work than with the 40 wt oil. I get about the same pressures with either oil.

I'm trying to sort out some observed data and will probably be left with more questions. We assume that thinner oils more easily uptake btu's ..which follows with our normal perceptions of other common observations. What we don't know (or at least I have trouble defining) is whether the apparent oil temps that we see, either higher or lower, are the function of heat uptake/rejection or interactive properties that promote heat creation.

That is, if we accept that thinner oils uptake and reject heat better than thicker oils ..fine. That belief sort of mandates that you also would assume that thicker oils would always be lower in temp since their uptake of btu's would be reduced. You sorta can't have one school of thought without adopting the other (there's naturally more to this ..but let's save space). So we have to also consider that there is some reactive/interactive component that is in place besides simply heat transfer characteristics. Does oil viscosity merely reflect/indicate conditions temperature-wise, or does it cause it???


We commonly see many applications where a "lubricant" is more of a coolant. Industrial compressors/blowers, automatic transmissions, etc. where the actual lubricant has little tolerance for high shear/high stress conditions beyond a narrow temp/flow range.
 
quote:

Gary, when I said "high load" I meant load and not high RPM! Put your car in second gear and do some rolling starts with full throttle. That's high load! Let's see what your rod and crank bearings will "say." Go high speed and let drag and air resistence cause high load on the engine. Sure, high RPM and high load can occur together, but not likely at the local street-legal speeds.

Well, would 75 in a minivan climbing the mountains of central PA on I-80 qualify?? I don't think my oil temp broke 160F. Granted I haven't done any "Fast and Furious" action in my minivan ..but
dunno.gif
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(visions of the black vantriliquist on Police Academy 3, with Taiwanese Kung-Fu movie poor lipsynch):
"I will accept your challenge!"
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The minivan ..so obedient ..so docile ..so unnoticed ..so under estimated...it asks no praise ..and makes no apologies.
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quote:

Originally posted by 427Z06:
When testing a non-friction modified 15w40 HDEO and a friction modified dino 5w20 in particular engines, my experience has been that when they have non-friction modified 15w40 HDEO in them, they reached operating temperature is less time. My guess is, they were burning more fuel to maintain the same idle and moderate acceleration levels, thus the engines were generating more heat.

I just like to add to this, that once up to temperature, the friction modified dino 5w20 stabalized at a lower temperature than the non-friction modified 15w40 HDEO under moderate driving conditions.
 
quote:

Originally posted by AEHaas:
And this is why oil is so difficult. At 100 C two oils may have the same viscosity. The same oils may differ widely at 10 C and below and may differ widely at 150 C and above. This is why we have to compare viscosity only at a certain temperature.

The viscosity can also be defined by the application. My Ferrari runs much cooler with the 20 wt Mobil 1 than the 40 wt Shell Helix Ultra. My operating viscosity is about the same as the "thicker" oil at the higher temperature. My oil is NOT thinner driving to work than with the 40 wt oil. I get about the same pressures with either oil.

aehaas


It would make a lot more sense to show oil viscosity specs as a curb on a temp vs cSt graph rather than just 2 data points at 40 and 100 C and extrapolating a VI from those 2 points.
 
Dr.A - Just because the pressure is the same doesn't necessarily mean that one oil is thinner/thicker/the same, than the other. The pressure gauge reads from the pump and relief valve operation, and the oil may be thinner at the cams, rings, bearings, where heat and pressure have great effect.
 
If the oil leaves the sump at 180 F and maybe picks up 10 degrees on its way to the bearing it would be at 200 F at the entrance to the bearing. If if then picked up 50 F there it would end up at 250 F.

The thicker oil starts out at 240 F at the sump, picks up 10 and then 50 more through the bearing. It ends up at 300 F.

aehaas
 
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