Castrol R is NOT compatible with mineral oil.
Very good racing oil but requires complete flushing of engine before first use.
Last I used it was about 10 years ago in Asia old stock from shelf.
If my memory serves me right Castrol R did not have much cleaning agent and not really recommended as a `daily' oil.
Given the advance of synthetic oil I really doubt Castrol still makes it.
Good memory though
I use synthetic when I could afford it. But had my dislike for the early version of Mobil 1 as it always seemed to lose its shine on turbo-engines after the first 1000-1500 miles.