I was just trying to find some quantification on how much this may help fuel economy. Its obviously one of those YMMV scenarios. For me, I do a lot of short runs. My commute is 7 miles, to do a lot of shopping its less than that. Warm up is super important. For others, it may not be.
Anyway, I did find this article about oil viscosity and fuel economy. It basically says the difference between 10W30 and 15W40 is 1% in fuel economy. No specifics of how things were tested were shared. This is probably considering the engine is already warmed up. Therefore, fuel economy before warming would be even larger, thus warming is more important.
Research Shows Oil Viscosity Affects Fuel Economy - Article - TruckingInfo.com