Ok I agree but this is the way it was explained to me I aviation engine school decades ago:

The higher the viscosity the longer it takes for it to return to the oil pan. A high volume pump can suck the pan dry in a racing situation as the oil is draining back through the valley pan area into the pan. Which is why dry sump systems exist as well as large volume oil pans. Dry sumps don't rely on gravity to return the oil to the pan. Large oil pans hold so much oil they can't run dry. And, I'm guessing this car will never see below 32 degree weather so a multi viscosity oil really doesn't matter. I agree with the oiling recommendations listed above though. If it were my engine I'd run something with a 30 or 40 weight as the second number. I prefer full synthetic. Its a hell of a lot cheaper than a engine rebuild. Oil is one if the things it doesn't pay to be cheap on. Like brake pads...think about it.

Just my pennies,
CC