While the debate can go on forever I think it's universally agreed that castor is necessary in some engines and the OP's old OS 20 meets all the criteria. It's got an iron piston, unbushed rod and a bushed crankshaft bearing. Also the Cox engines with the ball jointed rod need castor. As far as the debate goes, I usually refer to [link=http://www.go-cl.se/castor.html]this[/link] page for some very good reading on the subject by a couple of experts in the lubrication field.

Personally, with one exception, I've only ever used all castor in the last 50+ years. At first it was because it was the only suitable oil available and later because it's utterly reliable and much cheaper than synthetics (I've always mixed my own fuel). The exception was a new engine that was CL stunt specific (and not all that cheap) where the manufacturers specified only synthetic at 16% which was far from my normal blend but I figured they knew best. That left me in a quandary because I'd never fully trusted the usual synthetics so I ended up using Mobil gas turbine oil which worked very well but eventually I went back to the castor because the jet oil was quite expensive plus I was advised by one of our top flyers using the same engine to use 25% oil and castor. On castor the engine built up with carbon fairly quickly so I'll have to try a blend with a little synthetic in the package. Not all engines carbon up as quickly though, my Enya 60X has had a lot of running with all castor and there's barely a trace of carbon in it.