Spark Plug and Oil? Confused?

[chgolatin2]

So what oil will my both cars be happy with, the Porsche S and the BMW 540? Should I just use 10W40, 5W40 or 5W30? The previous owner used 15W50 but the car was driven in an all summer weather condition "Florida" I live in Chicago, Illinois weather changes dramatically and on the winter I dont drive my Porsche just in the summer but from time to time I tend to turn on the Porsche and let her run for the occassional winter warm ups.

I would like to stick with the Amsoil brand of oil, they offer a few options 10W40 high performance, 5W40 european formula and 5W30 high performance. I would like to use the same oil on both cars.

Thanks to all in advance! Robert (Chicago)