The Following is from jdub over at supramania. Knows alot about oils
A 20W-50 will increase oil pressure and keep the bearings from touching the crank journal/rod end - False
A thicker oil will increase oil pressure in the 7M, but only to a point. The design of the 7M has the oil relief valve set at 63-71 psi...over this oil is simply routed back to the pan. More importantly, the oil cooler valve on the stock filter head operates at 40 psi and you have a controlled "leak" from the oil squirters on a turbo motor. Pressure is resistance to flow...a 20W-50's increase in pressure means that it does not flow as well through the bearings or to the top of the motor. Considering a 20W-50 oil is 3-4 times thicker cold than a 5W-30, it's significant. This means a couple of things:
- On a cold start, you will have restricted oil flow where you need it the most.
- The oil relief valve will cut the flow further by opening at the higher pressure and dumping oil back into the pan.
- It will take longer for the oil to get to the cam shafts...the small diameter hole in the middle does not facilitate flowing high viscosity oil. Since the oil flows from front to rear on the cam shafts, it is going to take the #6 cylinder valves the longest to get the oil flow needed.
- The oil squirters spray pattern on the cylinder walls will degrade with a high viscosity oil...especially cold. It's simple...higher viscosity, lower velocity.
Oil pressure has little to do with the way an open bearing (like rod & main bearings) works. Like I said, these bearings depend on hydrodynamic lubrication to function...saying increased pressure will improve this is a very basic misunderstanding of how the bearings really work. The ONLY time I could see using a 20W-50 from a bearing standpoint is if you've built the motor for it by opening up the bearing clearances to get the flow...you will see some very experienced engine builders do this, mostly on race cars that get re-built every season anyway.
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