There is no reason that the oil viscosity would have caused a problem, the thinnest the oil gets is SAE40 when it's warm.

It is possible that there was sludge in the engine that made its way into the oil pump and pickup, more likely that the oil pickup has come loose or other oil-pressure degrading issue conincident with the period in which you ran the Mobil oil.

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Gone to the dark side

- Jeff