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Synthetic vs Organic
I have found that the synthetics (and seen data) where the synthetics
"flow" much better than organics. I think this translates into a thinner
oil at a given temperature. I don't completely understand this as I
would think that viscosity is viscosity and for exammple 10W-40 organic
would be similar to 10W-40 synthetic.
Some proof of my own: I have an old 79 F150 pickup with 140K miles and a
pretty tired engine. If I get off the highway on a hot day and stop at a
traffic light I can watch the oil pressure gage slowly drop down to the
bottom of the "normal" range (pretty scarey if you don't catch it). I
will put the transmission in neutral to allow the engine to idle a few
hundred rpm higher and the oil pressure returns to 1/4 normal range (at
highway speeds oil pressure is 3/4-7/8 normal range). Now the oil part.
I have noticed this oil pressure thing is much more pronounced with
narrow viscosity synthetcs vs. the organics (ie. 10W-40 or 10W30) but
with the wide viscosity synthetics ie. 5W-50 Syntec, 15W-50 Mobil it is
better than with the organics.
You might try a higher viscosity synthetic like 5W-50 Castrol Syntec
(this is what I currently run in all my cars) or the 15W-50 Mobil.