I use 100% synthetic exclusively, generally sticking with a 30:1 ratio.
Castor has historically been used for it’s superior lubricity, but I believe that modern synthetics have certainly equaled if not surpassed castor.
Castor is also a vegtable oil and will spoil like any other cooking oil so if you do elect to stick with castor buy it in small quantities, as needed and never leave it sitting on the shelf for extended periods.
My motors are clean, well lubricated, with much less oil smoke.
The only motor I’ve ever ruined was leaned to the point it detonated and imploded.
The one thing I can say about castor is that it smells great. I love the smell of castor!