The pharmaceutical castor is okay to use, but does cost more. The last quart I bought was Sig castor at $7 at the LHS.

W8YE and I replaced bearings in a Saito .80 that a friend was flying, and he added drug store castor to his fuel for a long time. The retainer had failed on the bearings. Otherwise, the inside of the engine and the actual bearings were completely rust free. We were a little surprised at how good the engine looked from this grade of castor, and amount of metal that had circulated in the engine from the retainer.