Ken,
Sig sells high quality castor oil.
For an excellent discussion on old engines, oil and fuels, from here go back to the Stunthanger home page and scroll down from "Open Forums" that we are in now. Keep scrolling down until you get to "Engine set up tips.". The second topic "Engine tuning tips" by Randy Smith covers the old engines (Fox .35, OS Max S, etc.) and what oil percentages to use to keep them running, glow plugs, and other tips to get them to run well and safely.
Joe Ed Pederson
AMA 103434