I just did the cocktail tango tonight and ALL whirring gear noises are gone...yup all! I do have a clanking on decel between 2-1k rpms but usual gear noise is now silent. Oh, but I decided to use the heavyweight shockproof.
I was using Mobil 1 75-90 before and now with the cocktail, and driving a while to warm it all up, it shifts the same as with the Mobil 1, nice and easy as before but now silent.
About draining and not being able to find the Castrol, the most important ingredient is the shockproof and syncromesh. I to couldn't find the Castrol but heard great things about Lucas 80-90 so that's what I used.