I can give you an extreme example of the benefits of lower viscocity oil:
In air cooled VW drag racing circles circa 1990, for the cars running in Pro turbo, Pro Stock, and Super Street heads up classes, it was a given that your gearbox needed to rebuilt after each event. The ring and pinion set in the type 1 gearbox is notorious for cracking from drag racing starts when putting down about 300hp or more. It was pretty common to see the fastest pass of the day in the final round, even though it is held in the heat of the afternoon. Typically one or both competitors pulled out all the stops, throwing every trick in the book at the car's set up and tune, and drained the transaxle of all the oil just before the final
the word is they'd decrease ET by 1/10th of a second on a 9.70 1/4 mile ET having little or no oil in the transaxle (a 10th is a big difference in heads up drag racing)
**obviously you wouldn't do this in your street car**