I have found this to be very good. It is a mixture of degummed natural castor oil and synthetic ester based oil. It is intended to be used as a premix in petrol. It is totally miscible in the methanol used in RC engines. I make 18% fuel with no nitro methane.

https://shop.motousher.com/products/fully-synthetic-oil-2-stroke-castor-927?variant=10656491175980&currency=INR&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=Cj0KCQiA95aRBhCsARIsAC2xvfz8MDl6AJ7QtTWJ81RPcCLxFzhpJHKAlvgGmQ3QnRjPoWzC3T8IBcIaApQdEALw_wcB

How long have you been using it? How many tankfuls has an engine done? Have you opened the engine before and after to check wear, pitting?