Don't fret much over the oil pressure fluctuations. Oil pressure changes depending on any number of variables:

1. Heat - the oil pressure drops as the oil heats up.
2. Engine speed - the oil pressure drops at idle.
3. Age - the oil pressure drops as the oil ages.

Unless you're using synthetic oil in the engine, I vote for using anything between a 10W-30 and 20W-50 weight during the heat of summer. I wouldn't fret too much over the weight of the oil, as long as it's reasonable. You're correct with your logic on how to interpret the two oil weight numbers.

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95 E320 Cabriolet, 169K