Almost, but not quite :msp_biggrin:. Castor is still in widespread use by kart racers, model engine groups, roadracers, MX, etc. When you say esters wil do pretty much anything that castor will do that is correct. The one thing they wont do is, as you mentioned, grow polymer chains in extreme heat. Esters break down in this environment, like most any other lubricant. This nasty gummy layer of varnish you talk about I have not seen in decades. The de-gummed racing castor available now is quite clean. I ran my TZ250 for an entire season on castor based lube. It was pulled down after every race weekend and there were never any deposits that couldn't wiped off with a rag. To show the amount of trust I had in castor oil, I was using it to protect an engine that had two piston/cylinder sets that cost $600 each to replace. The crank set was $1800 by itself. The only thing I had to do was replace the pistons at the end of their scheduled life. I also trusted the oil to keep my hind end safe. A top end siezure while headed into the backstraight dip at Road Atlanta at 160MPH is a rather undesirable thing.
I'm not about to tell anybody what oil to run. But I can share my own personal results with oils that have worked for me. Ever since Stihl introduced the Ultra synthetic my OPE has been run run with a pre-mix of Ultra at the recommended ratio, with one ounce per gallon of Klotz Super Techniplate added. My top ends have stayed clean as a whistle.