Quote:
Originally Posted by X25
I tried 5W-30 (replacing 0W-40) on my supercharged Miata today, combined with removed thermostat, and my oil temps stabilized at about 20-30 degrees lower, around 250-260 degrees F. I'm not sure which one helped more, but I am definitely pleased.
On my way back, I was monitoring my truck's oil temp and pressure, and especially the pressure was definitely way too high. At 2000 RPM, it was already making about 55 PSI, and it doesn't climb much higher than that (perhaps up to 65 PSI or so), suggesting I am just recirculating oil through bypass valve at as low as 2000+ RPM! Per Bob, 20 PSI at 2000 RPM is perfectly fine. Since the pump flow is linear to the RPMs, you expect a somewhat linear increase in oil pressures as the RPMs climb. If it doesn't, it means the oil is just flowing through bypass valve, lubricating nothing, and heating itself. It also hit as high as 240 degrees F during the uphill section. It is obviously not alarmingly high for a truck that is towing, but I wonder if I can reduce that. The oil in use is Rotella T6 5W-40, which is actually significantly thicker than gas 5W-40 oils. Does it do its job? It probably does; Rotella never let me down on any car. Is it optimal? Far from it. The user manual calls for 5W-20, and that's what I will switch to next week (Mobil1 5W-20 EP to be exact). I am looking forward to towing my car again, and checking how oil pressures and temps fare. I also expect a measurable (1+ MPG) fuel economy with the change.
In short:
RAM 1500 Hemi --> Mobil1 5W-20 EP
Supercharged Miata --> Mobil1 5W-30 EP
Camaro 1LE --> Mobil1 5W-30 EP
I have a stockpile of 3x5-quart Mobil1 0W-40. I can sell them for $20 each if anyone is interested, though I suppose I'm not the best salesman by stating I think 5W-30 is the better choice :P
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X25, What does the "EP" signify in your oils listed above? Is that for the European Blend?
Also as I've been reading through these oil threads and taking in the discussion about oil pressures it seems to me that if we just look at the type of pump that the oil pump is, it starts to also shed light on why higher pressure does not mean better lubrication. Oil pumps are usually a gerotor style which are I believe are positive displacement pumps. Link below describes how gerotor pumps work and specifically states "Constant and even discharge regardless of pressure conditions"
http://www.pumpschool.com/principles/gerotor.asp
Given how positive displacement pumps operate they are going to pump out the constant flow rate (i.e. volume) at a given pump speed no matter the pressure (until something breaks at a high enough pressure which is why they have a bypass valve).
So a higher pressure just indicates a higher resistance to flow. A lower pressure means a lower resistance to flow. This has been something I have questioned for a while when everyone says how they are happier with a higher oil pressure at idle after switching oil grades. To me all that tells me is the new grade of oil is harder to move through the oil flow passages thus creating more backpressure on the pump and a higher oil pressure.
Obviously you don't want to see "0 psi" for oil pressure as there should always be some oil pressure when the pump is working. But again to me higher pressure just means harder to pump.
Also pumping a constant volume of oil (which is what positive displacement pump will do at a given speed) but at a higher pressure just means you are putting more heat into the oil from pumping alone. This seems counterproductive as we are always trying to minimize the heat gain in the oil.
I guess that is just my engineering thoughts on this whole issue. I did not read the Bob The Oil Guy pages to learn more about how the different viscosity's work but just looking at fluid pumping principles with positive displacement pumps by itself I have not always believed that higher pressure = better oil lubricating.