From the site
http://www.westonuk.co.uk/index_045.htm as this text:
...fuels contains what we believe to be the best synthetic oil...
This word "belive" can mean they are not sure the syntetic oil are reliable as the castoroil.
And the text from other site:
P.S. I have noticed one thing over the past few years though. The people I fly with who use Prosynth seem to spend more money on bearings than I do.
Jens Eirik