The "30" in "5w30" is the hot viscosity. In cold temps 5w20 and 5w30 should have the same viscosity. The difference would be as the oil is warmed up, like in a running engine. The 5w30 should be a little thicker, so it should (as you mentioned) have a slightly higher oil pressure at operating temp.Wouldn't 30 weight be thicker in viscosity and therefore increase pressure but also only in cold Temps?
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The only thing I can think of to explain what the OP is saying would be the fact that the Pentastar 3.6 is supposed to have a two stage oil pump set up, where it brings on the second stage when it wants more oil pressure. So the oil pressure can go from around 45 psi to around 90 psi depending on whether or not it is running the second stage oil pump.