It's not only about fuel consumption. Thinner oils started to be used more when engines became taller, i.e. the more common use of OHC engines.
It was difficult to pump a cold thick oil to the top of these engines on initial start up, therefore, a lot of them suffered premature camshaft failure. In order to get the oil to the top of the engine on start up the oil used became thinner.
Back in the day with OHV engines, think Ford Escort, Vauxhall Viva, Hillman Hunter, Avenger etc, all these cars had OHV engines and 20/50 went into everything without a second thought.