Brother of The Benz, Robert Boyer
As you can read, most of us use Mobil I, 15W50 for the higher temperatures.
Here in Houston, it satisfies the winter temperatures as well.
Many use this oil for the extended oil changes as well as it's superior lubricating properties.
For me, I do the 3500-4000 mile oil and filter change regardless of the base of the oil.
My 103983 engine with 177,000 miles has never had the engine serviced, not even valve stem seals.
Clean oil is the name of the game, not it's base.
In reply to the vibs you felt in your engine, could you have over filled the crankcase causing oil to be slung up by contact with the crankshft?
There should be no appreciable amount of engine performance change between the two oil weights.
Return to the heavier oil and keep accurate records.
Happy Trails Beep Beep from The Spiderman in Houston!!!