It seems many oil companies were big on pushing heavy oils in the 1980's 10w40 and 20w50 weight oils were much more commonplace as a matter of fact my Honda Civic ran Castrol GTX 20w50 in the summer and 10w40 in the winter. Were lighter weight viscosity oils not up to the task back in the 1980's it seems 5w30 and 10w30 oils were not recommended as much? .. (Note I am referring to the 1080's as this is when I grew up I can't comment much on earlier time)