I think you're right, if your short journeys mean the oil never really warms up properly, a 30 should be better than a 40. However, the only way to decide is stick it in and see, listen to the engine and feel if it seems better or worse. All engines are different, oils are different.
You may find a 40's better first time you go over ten miles on the motorway. I once tried 5-30 in my Probe, which is what Ford recommend, but soon went back to 5-40, it just felt better. 100000 miles later I guess I was right and Ford were wrong.
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