Originally Posted By: Gokhan
Originally Posted By: Shannow
Originally Posted By: Garak
Originally Posted By: Gokhan
Excessive oil pressure
Reduced oil flow
How do we define "excessive" oil pressure? Where are we getting reduced flow?
And where in any engine does this impact lubrication...empirical evidence only, not motherhood statements about "obviously" and "feeling".
Some effects of increase in viscosity:
- Excessive heat generation resulting in oil oxidation, sludge and varnish build-up
- Gaseous cavitation due to inadequate oil flow to pumps and bearings
- Lubrication starvation due to inadequate oil flow
- Oil whip in journal bearings
- Excess energy consumption to overcome fluid friction
- Poor air detrainment or demulsibility
- Poor cold-start pumpability
Source:
Viscosity: a lubricant's most important characteristic -- Parker Kittiwake
Let's compare the two viscosities being discussed in this thread rather than hyperbolic claims loosely based on some PDF that happens to support one's spurious posit.
Mobil 1 15w-50 has the following viscosity characteristics:
Viscosity @ 100ºC, cSt (ASTM D445) 18
Viscosity, @ 40ºC, cSt (ASTM D445) 125
Mobil 1 EP 0w-20 has the following viscosity characteristics:
Viscosity @ 100ºC, cSt (ASTM D445) 8.6
Viscosity, @ 40ºC, cSt (ASTM D445) 44.9
Let's plug these puppies into a visc calc. Yes, we know the low temperature difference is going to be significant, but of course the thicker oil will, due to more viscous shear in the bearings, heat up quicker.
At 115C, the 15w-50 is 13cSt, the same viscosity as the 0w-20 at ~81C. As the heavier oil gets hotter, it is going to get thinner, so even if there's a 10 degree variance (which I wouldn't describe as "significant" as per your earlier language and I would be surprised if it was even that big) then the variance between the two oils in viscosity is actually less than one would be led to believe.
Let's say we DO have a 10C difference however. And let's say our operating oil temperature with our 0w-20 is 110C. That gives us 7.1cSt for the 0w-20, and if our 15w-50 increased oil temperature to 120C, then it would be 11.8cSt, or the same as the 0w-20 was at 85C, a far cry from "significant" and would be well within the temperature and subsequently viscosity variance experienced during seasonal use.
Now, that's not to say that any of this would be of benefit to your average application, but it isn't going to spell disaster either. The reasoning that drove Ford to spec 5w-50 for their engine in track use applications that otherwise spec'd 5w-20 is the same reasoning that drives GM to recommend this 15w-50 for use in their vehicles used under similar operating conditions. That relatively small buffer of viscosity can be the difference between a bearing getting wiped or not, which depends on HTHS, a part of this we haven't discussed.
15w-50 may be less than ideal for a particular application that specs 0w-20. And it might be a bad idea if that application experiences -20C. However, under appropriate operating conditions, other than some minuscule gas mileage and power penalties, it isn't the fire and brimstone being peddled here to support one's personal vendetta.