-And, the discovery was an accident!
It took place in 1877, at Crossley Works in Manchester. An operator who was cleaning a gas engine that jammed by combustion deposit solidification each night. He mistakenly filled up the engine with colza lamp oil instead of cylinder lubricator. As a result the engine free to turn the next morning.
The significance of this was soon realized and oil companies by experiments cleaning behavior could be obtained through adding roughly 10% of vegetable or animal oils to archive the cleaning effect in engine. Blended oils also had lower coefficient of friction than mineral oils thus making suitable for bearing to run at higher speeds.
Exclusively, the blended oils were for racing car engine. Gradually they spread into private car arena as blended oils formed smaller amounts and softer deposit making cleaning easier, decoking also less frequent.