IMO there is NO substitute for Castor oil.
Let someone else/use praise the benefits of Synths.
When the engine conditions become extreme the castor provides that extra bit of protection that saves the engine..
Pharmacy /medicinal castor works 'fine', yesss it gums more, but it's a Small price to pay for a useable engine.
SIG does (did??) sell quart jugs of their finest Castor, asking a very fair price + shipping $.. very nice.
Recently dug out a few old Fast Combat motors, 2 were messed (bores and rods) from running on Klotz laced fuel instead of Castor based , not too many runs either according the the storage notes.
(IIRC..it was the popular 'new' stuff .. back then)
Everybody makes mistakes
Trick is in not repeating the more popular ones :-)