Depending on the application single grade oils are just as acceptable as multi-grade oils. Prior to about 1964 everything was single grade. Certain engines, like the Detroit Diesel two-strokes in my boat, use a single grade 40 oil, and are not permitted to run on multi-grade at all. Up until about twenty years ago there were no multi-grades acceptable for diesel engines in commercial vehicles.
Synthetics can achieve multi-grade certification without the use of viscosity index improvers that conventional oils must use to achieve the same thing, which is another advantage of synthetics. VI improvers are in and of themselves, not a lubricant, yet they displace the total amount of lubricant in any given quantity.