Pharmaceutical-grade castor oil sold as a laxative works great. It's completely unprocessed and has no additives. It isn't even de-gummed, so a small amount added to your fuel will allow a thin film of varnish to form over time, sealing scratches in the piston and cylinder. It's expensive vs. industrial-grade castor oil though, since it has to pass extra purity tests, but I haven't found unprocessed industrial-grade castor oil in a long time -- it's all degummed nowadays, because people are willing to sacrifice a little bit of extra lubrication in exchange for lower maintenance.

That being said, while the pharmaceutical-grade stuff costs more per mL, you really don't need much. I run Byron's Gen2 fuel with 11% oil premixed, and I add 43mL of pure castor oil per gallon to raise the oil percentage 1%. I can spike about 2.5 gallons of fuel using one small bottle of pharmaceutical-grade castor oil from my local drug store.

But yeah, don't use castor oil intended for skincare unless it specifically says it contains nothing else.