Oil Temp @ Pressure ??

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Jul 8, 2002
Lake Anna, VA
Well After an AutoRX run I switched to Chevron 10W-40 for a rinse phase. Yesterday I drained out the Chevron and filled back up with Schaeffer's 15W-40 Blend. Well I noticed something different with the Schaeffers over the Chevron. After driving 30 mins on the highway my oil temps were the same with both oils around 200-210 degrees but I noticed a vast difference in oil pressure between the two oils. The Chevron 10W-40 would drop to 30psi at hot idle, oil temp 200-210 while the Schaeffers would only drop to 40-42psi at same temp hot idle. What could cause the oil pressure difference between the two? Both oils produced the same pressure under load 50-55psi. Vehicle in question is a 2000 Jeep Wrangler 2.5L I4 lifted, locked and ready to rock! Very heavy TJ loaded down with many aftermarket accessories and has 4.88 gears which makes the little 4 squirells turn 3000rpms at 70mph.
A couple of possibilities: Are you comparing the pressure on well used sheared down Chevron with fresh Schaeffers? The Chevron and Schaeffers and Chevron could have quite different viscosities at your operating temperature and still both qualify as xW-40 oil. Since you are operating near 100C (212F), perhaps you could look up the actual viscosities on their websites. That would tell you what they intend the vicosity to be, not necessarily what your batch was.
Probably a 15w40 is constructed with a higher 100C viscosity than a 10w40. That would explain the difference at idle. Hot under load pressure is the same with both oils, probably because in both cases the oil pressure relief valve is opening up, without which maybe one would run 60 psi and the other 70.
Also, consider where the oil pressure sender is located. If it is the same as the 4.0L I-6 Chrysler engine, you're measuring the oil pressure before the filter. So any additional resistance given by the filter would add to the oil pressure.
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