Quote:
Originally Posted by ah-kay
Oil spec has nothing to do with pressure. Liquid is uncompressible so even water would give the same pressure.
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It has nothing to do with the compression, it has to do with viscosity. Thinner oil slings out of the bearings, journals, and any "wear" areas in the engine, higher leakage rate results in lowered system pressure. The oil weight corresponds to a given viscosity, shear rate, and film thickness. Thinner oil in a worn engine will result in lowered oil pressure compared to a higher viscosity. It isn't a huge amount, but it does produce a change in the overall system pressure.